STEEL 


AND  ITS  HEAT  TREATMENT 


BY 

DENISON    K.  BULLENS 

* » 

Consulting  Metallurgist 


FIRST  EDITION 

FIRST  THOUSAND 


NEW  YORK 

JOHN   WILEY   &    SONS,    INC. 

LONDON:  CHAPMAN  &  HALL,  LIMITED 
1916 


Copyright,   1916 

BY 

DENISON    K.    BULLENS 


.•1/Z  rights  r?serocd 


THE  SCIENTIFIC  PRESS 
ROBERT   DRUMMOND  AND  COMPANY 


IN  MEMORY  OF  MY  FATHER 


331168 


PREFACE 


MODERN  Heat  Treatment  should  be  considered  as  an  art  or  trade, 
since  it  certainly  requires  knowledge,  skill  and  judgment  for  its 
proper  performance.  These,  in  turn,  necessitate  at  least  some  knowl- 
edge of  heat,  of  steel,  and  of  the  effect  of  heat  upon  steel.  And  all 
three  factors  are  linked  together  by  the  "  human  element."  The 
author  has  therefore  endeavored  to  bring  together  the  theoretical  and 
practical  sides  of  the  general  subject  of  steel  and  its  heat  treatment 
in  such  a  manner  as  will,  he  hopes,  be  understandable  by  that 
"  human  element." 

It  has  been  the  author's  attempt  to  make  the  chapters  dealing 
with  the  heating  problem  more  of  a  "  heat  talk  "  than  of  a  "  furnace 
talk";  of  heat  application  rather  than  details  of  construction;  of  the 
importance  of  the  human  element  and  scientific  efficiency  rather  than 
the  elimination  of  the  human  element  through  scientific  management; 
and  finally,  of  viewing  the  heating  problem  as  an  engineering  prop- 
osition, adapting  each  fuel  to  proper  furnace  design  and  operation 
to  meet  the  requirements  of  the  problem  in  hand,  and  by  so  doing 
aim  for  the  adoption  of  the  standard  heating  unit  in  terms  of  finished 
product — "  the  cost  of  a  unit  of  quantity  of  given  quality." 

He  has  attempted  to  make  as  practical  as  possible  those  chapters 
relating  to  steel  and  the  effect  of  heat  upon  steel.  Theories  have  been 
advanced  only  so  far  as  has  been  thought  necessary  for  a  clear 
understanding  of  principles.  Wherever  possible,  illustrations  in  the 
form  of  photomicrographs  and  charts  have  been  given.  The  data 
given  under  the  various  types  of  heat-treated  steels  have  been 
checked  as  far  as  possible  and  every  effort  has  been  made  to  be 
correct. 

v 


vi  PREFACE 

To  the  many  friends  who  have  aided  him  in  the  preparation  of 
this  book  the  author  would  express  his  sincere  appreciation.  Effort 
has  been  made  to  give  due  credit  for  cuts  and  data  at  the  proper 
place,  and  for  such  as  may  not  have  been  made,  acknowledgment  is 
hereby  rendered. 

DENISON  K.  BTTLLENS. 
PHILADELPHIA, 

October  1,  1915. 


CONTENTS 


CHAPTER  PAGE 

I.  THE  TESTING  OF  STEEL 'I'- 
ll. THE  STRUCTURE  OF  STEEL 16 

III.  ANNEALING 39 

IV.  HARDENING 65 

V.  TEMPERING  AND  TOUGHENING 96 

VI.  CASE  CARBURIZING  . 112 

VII.  CASE  HARDENING:     THERMAL  TREATMENT 154 

VIII.  HEAT  GENERATION 173 

IX.  HEAT  APPLICATION 192 

X.  CARBON  STEELS 229 

XI.  NICKEL  STEELS 257 

XII.  CHROME  STEELS 295 

XIII.  CHROME  NICKEL  STEELS 306 

XIV.  VANADIUM  STEELS 335 

XV.  MANGANESE,  SILICON  AND  OTHER  ALLOY  STEELS 344 

XVI.  TOOL  STEEL  AND  TOOLS 357 

XVII.  MISCELLANEOUS  TREATMENTS 386 

XVIII.  PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS.  .  .   408 


vii 


STEEL  AND  ITS  HEAT  TREATMENT 


CHAPTER   I 
THE  TESTING  OF  STEEL 

Growth  of  Heat  Treatment. — Probably  no  one  division  in  the 
metallurgy  of  steel  has  taken  such  wonderful  strides  in  recent  years 
as  has  the  art  of  heat  treatment.  Twenty  years  ago  the  scientific 
knowledge  and  technical  application  of  heat  treatment  were  but 
very  limited.  Such  as  it  was,  it  usually  consisted  in  "  heating  to  a 
red  heat  "  for  annealing,  or  perhaps  the  instructions  called  for 
"  harden  at  a  bright  red  and  temper  to  a  straw  color."  Then  it 
was  an  art  guarded  with  much  secrecy  and  confined  for  the  most 
part  to  makers  of  tools  and  a  certain  few  specialties. 

Practically  all  alloy  steels  require  treatment  of  one  sort  or  another. 
In  the  "  natural  "  state  very  few  steels  present  their  full  value,  so 
that  heat  treatment  is  not  only  advisable  but  often  mandatory. 

Necessity  for  Heat  Treatment. — Take  for  example  the  steels 
used  in  the  automobile  industry.  The  frame  requires  resistance  to 
vibratory  stresses  occasioned  by  rough  roads,  as  well  as  strength  and 
toughness.  Rear  axles  must  have  great  tortional  resistance;  front 
axles  must  withstand  vibrations.  The  steering  parts  must  be 
strong,  tough  and  without  brittleness;  the  springs  must  neither  sag 
nor  break.  Crank-shafts  must  be  able  to  resist  impact,  besides  being 
stiff.  Gears  are  subject  to  wear  and  must  be  capable  of  withstanding 
this  action  if  a  smoothly  running  transmission  is  to  be  had.  And 
so  each  separate  part  might  be  named,  all  having  a  more  or  less 
severe  duty  to  perform  and  requiring  steels  possessing  various 
degrees  of  strength,  toughness,  resilience,  endurance,  shock-resisting 
and  wearing  qualities. 

Testing. — These  various  combinations  of  static  and  dynamic 
strength}  are  obtained  by  adjusting  and  correlating  both  the  chemical 


2  STEEL  AND   ITS  HEAT  TREATMENT 

composition  and  heat  treatment  of  the  steel.  Certain  chemical 
components  intensify  the  static  properties  of  the  material;  others 
may  affect  the  dynamic  qualities.  Thus  by  coupling  with  a  steel  of 
suitable  chemical  combination  the  proper  heat  treatment,  there 
arises  a  product  with  physical  properties  most  adapted  for  the  work 
in  hand.  Similarly,  having  once  produced  a  suitable  article,  it  then 
remains  to  duplicate  it.  To  this  end  all  rational  heat  treatment 
must  be  aligned  and  standardization  of  results  be  obtained.  In 
order  to  accomplish  these  specific  requirements,  the  influence  of 
definite  chemical  composition  and  definite  treatment  must  be  known, 
as  will  be  described  in  later  chapters.  The  guide  to  this  work  is 
frequent  and  constant  testing,  and  a  definite  knowledge  of  the  vari- 
ous components  should  be  possessed  by  every  heat-treatment  man. 
Thus  we  may  say  that  the  purpose  of  practical  testing  is  (1)  to  sup- 
ply information  as  to  suitable  material  and  its  qualities  for  different 
purposes,  both  for  the  manufacturer  of  the  material  and  for  the 
designer  or  user,  and  (2)  to  test  the  specified  uniform  quality  of  the 
material. 

Stresses  and  Strains. — Testing  resolves  itself  into  a  determination 
of  the  strength  of  the  material,  which  in  turn  is  measured  by  the 
application  of  a  force  and  its  resultant  effect.  The  force  put  upon 
a  body  is  termed  the  stress,  and  the  deformation  resulting  from  that 
force  is  the  strain.  Upon  the  method  of  applying  that  force  depends 
the  nature  of  the  test.  Thus  we  may  conveniently  classify  such 
stresses  under  the  following  headings: 

A.  Steady  or  constant  loads — static  stresses; 

B.  Repeated  static  stresses  and  accelerated  stresses — fatigue 

stresses; 

C.  Suddenly  applied  loads — impact  stresses; 

D.  Repeated  impact  or  vibratory  stresses — dynamic  stresses; 

E.  Miscellaneous  tests  such  as  resistance  to  penetration,  wear, 

etc. 

Tensile  Strength. — The  most  common  test  for  static  strength, 
that  is,  the  strength  of  the  steel  under  constant  load  and  without 
shock  or  vibration,  is  the  tensile  test.  Thus  we  may  call  the  tensile 
strength  the  absolute  strength  of  the  metal  under  tension,  i.e.,  the 
force  actually  required  to  pull  the  metal  asunder.  A  standard  test 
piece  is  gripped  between  the  upper  and  lower  jaws  of  a  testing  machine 
and  the  total  resistance  to  rupture  is  measured.  Knowing  the  area 
of  the  cross-section  of  the  test  piece  and  the  load  required  to  break 


THE  TESTING  OF   STEEL  3 

it,  the  strength  per  square  unit  may  then  be  calculated.  The  tensile 
strength  is  usually  given  in  pounds  per  square  inch  (American), 
tons  per  square  inch  (British),  or  kilograms  per  square  millimeter 
(metric  system;  1  kg.  per  mm.2  =1422. 32  Ibs.  per  square  inch).  The 
accuracy  of  the  tensile  test  is  dependent  not  only  upon  the  conditions 
under  which  the  test  is  made,  such  as  the  rate  of  pulling,  alignment 
of  test  piece  in  the  machine,  etc.,  and  which  are  more  or  less  influenced 
by  the  human  element,  but  also  upon  the  metal  itself.  The  higher 
the  tensile  strength  and  brittleness  of  the  steel,  the  greater  the 
possibility  of  error;  differences  of  several  thousand  pounds  per 
square  inch  are  often  encountered  in  the  same  piece  of  high-tensile, 
heat-treated  steel,  even  in  the  absence  of  brittleness. 

Test  pieces  are  generally  taken  half  way  between  the  center  and 
the  outside  of  the  piece,  and  longitudinally  or  "  with  the  grain." 
Occasionally  it  is  necessary  to  take  tests  transversely  or  "  across  the 
grain";  in  this  case  the  results  will  be  lower  than  in  the  longitudinal 
test,  the  exact  amount  depending  upon  the  composition  and  treat- 
ment of  the  steel. 

The  static  strength  increases  in  direct  proportion  to  the  carbon 
content  of  the  steel.  For  the  ordinary  basic  and  acid  open-hearth 
steels,  without  heat  treatment,  Campbell  gives  the  following  formulae 
by  which  the  tensile  strength  of  such  steels  may  be  roughly  deter- 
mined. These  results  apply  for  steel  "  in  the  natural." 

Acid  open-hearth  steel : 

Tensile  strength  =  40,000 +1000C  +  1000P +xMn. 
Basic  open  hearth  steel: 

Tensile  strength  =  41,500+7700  +  1000P+?/Mn. 

In  these  formulae,  C  equals  each  one  point  (0.01  per  cent.)  of 
carbon  as  determined  by  combustion,  P  equals  each  0.01  per  cent, 
of  phosphorus,  Mn  equals  each  0.01  per  cent,  of  manganese,  and  x 
and  y  are  given  in  the  table  on  page  4. 

Elastic  Limit  (Tension). — The  term  "  elastic  limit  "  has  probably 
been  more  ill-used  than  any  other  common  technical  testing  name, 
with  the  possible  exception  of  "  hardness."  Among  its  many 
definitions  the  two  which  stand  out  pre-eminently  are  (1)  the  least 
stress  at  which  the  material  retains  a  permanent  deformation  or 
"  set  "  after  the  removal  of  the  stress;  and  (2)  the  least  stress  under 


STEEL  AND   ITS  HEAT  TREATMENT 


Percentage  of  Carbon. 

On  Acid  Steel. 

X 

Lbs.  per  Sq.  In. 

On  Basic  Steel. 

y 

Lbs.  per  Sq.  In. 

0.05  

not 

0.10  

80* 

130 

0.15  

120 

150 

0.20  

160 

170 

0  25  

200 

190 

0.30  

240 

210 

0  35. 

280 

230 

0.40  

320 

250 

0.45. 

360 

0.50  

400 

0  55 

440 

0.60  

480 

*  Beginning  only  with  0.4  per  cent,  manganese, 
t  Beginning  only  with  0.3  per  cent,  manganese. 

which  ductile  material  exhibits  a  marked  yielding — sometimes 
denoted  as  the  "  yield  point." 

The  determination  of  the  true  elastic  limit  should  always  be 
taken  from  a  curve  plotted,  using  an  extensometer,  from  a  series 
of  careful  observations,  as  otherwise  sets  caused  by  non-homo- 
geneity and  initial  stress  might  be  obtained  which  do  not  repre- 
sent the  plasticity  of  the  material.  This  method  of  determining 
the  elastic  limit  is  but  little  used  commercially,  as  the  amount  of 
labor  involved  is  too  great. 

The  yield  point,  or  commercial  elastic  limit,  is  obtained  by 
noting  the  stress  at  which  the  test  piece  first  begins  to  "  give  "  or 
elongate.  This  may  be  obtained  by  means  of  two  prick-punch  marks 
and  observing  the  first  signs  of  elongating  by  means  of  dividers  held 
on  these  points;  or  by  noting  the  drop  of  the  weighing  beam  or  halt 
in  the  load  indicator  ("  jockey  ") ;  or  by  means  of  the  general  appear- 
ance of  the  test  piece. 

In  its  practical  application  the  elastic  limit  may  be  called  the 
working  strength  of  the  material,  for  in  most  cases  the  steel  or 
machine  part  becomes  useless  when  strained  beyond  its  elastic 
limit.  This  is  particularly  true  of  automobile  construction,  in 
which  the  value  of  a  car  is  dependent  upon  the  correct  adjustment 
and  alignment  of  its  several  working  parts,  such  as  in  transmissions 
and  transmission  suspensions.  All  tests  given  in  this  book,  unless 
otherwise  noted,  refer  to  the  commercial  elastic  limit  or  yield 
point. 


THE  TESTING  OF  STEEL  5 

The  relation  existing  between  the  elastic  limit  and  the  tensile 
strength  is  too  broad  a  subject  for  discussion  here,  as  the  varying 
chemical  compositions  and  heat  treatments  exert  such  a  tremen- 
dous influence;  a  study  of  the  results  given  in  following  chapters 
will  show  a  proportionality  of  40  per  cent,  and  upward. 

Elongation. — The  elongation  is  measured  in  per  cent,  of  the 
original  test  section  and  is  commonly  the  amount  of  stretch  which 
will  occur  in  the  material  when  pulled  apart  by  tension.  It  is 
usually  measured  in  relation  to  an  initial  distance  of  2  or  8  in., 
or  100  mm.  when  the  metric  system  is  used,  but  other  specifica- 
tions as  used  in  Europe  give  a  definite  relation  of  original  gauge- 
length  to  the  thickness  or  diameter  of  the  specimen. 

Reduction  (or  Contraction)  of  Area. — The  reduction  of  area 
refers  to  the  area  at  the  point  of  rupture,  usually  reported  in  per 
cent,  reduction  of  the  original  area — that  is,  the  original  area  of  the 
test  piece  minus  the  area  of  the  smallest  cross-section  after  frac- 
ture; this  divided  by  the  original  area  is  the  percentage  reduction  of 
area. 

Ductility. — The  percentage  elongation  and  percentage  reduction 
of  area  are  a  measure  of  the  "  ductility  "  of  the  material,  usually 
varying  inversely  with  the  tensile  strength.  The  true  measure  of  the 
ductility  of  the  steel  cannot  be  taken  alone  from  either  the  elonga- 
tion or  reduction  of  area,  as  the  results  obtained  in  either  case  will 
depend  in  a  large  measure  upon  the  size  of  the  test  piece,  the  method 
of  testing,  etc.  Many  engineers  regard  the  reduction  of  area  as  the 
more  reliable;  this  is  offset  by  the  fact  that  many  steel  specifications 
make  no  mention  of  the  reduction  of  area,  but  particularly  specify 
the  percentage  elongation.  Ductility  may  also  be  defined  as  the 
amount  of  distortion  of  the  material  before  final  rupture. 

Compressive  Strength. — The  compressive  strength  of  material 
is  its  resistance  to  crushing.  The  test  is  generally  carried  out  upon 
a  small  cylinder  or  1-in.  cube  of  the  metal,  using  the  same  machine 
as  for  the  tensile  test.  Care  must  be  used  to  see  that  the  line  of 
strain  passes  exactly  through  the  axis  of  the  specimen,  and  that  the 
plates  above  and  below  the  piece  have  a  greater  resistance  to  pene- 
tration than  the  metal  to  be  tested.  The  application  of  the  term 
elastic  limit  is  similar  to  that  in  the  tensile  test. 

Torsional  Strength. — As  its  name  implies,  the  torsion  test  is  used 
to  determine  the  resistance  to  twisting.  This  test  is  very  largely 
used  to-day  for  automobile  steel  and  is  measured  in  inch-pounds 
with  the  amount  of  distortion  given  in  degrees.  The  elastic  limit  is 


6  STEEL  AND   ITS  HEAT  TREATMENT 

obtained  as  in  a  tension  test,  using  either  a  tropometer  or  an  auto- 
graphic attachment.  The  usual  comparison  is  by  calculating  the 
shearing  stress  in  pounds  per  square  inch. 

Endurance. — The  computation  and  understanding  of  such  static 
stresses  as  have  been  previously  outlined  are  comparatively  simple. 
The  requirement  to  be  fulfilled  in  designing  is  that  the  working  stress 
shall  not  exceed  the  elastic  limit  of  the  material,  whether  it  be 
in  tension,  compression  or  torsion.  Numerous  every-day  failures, 
however,  which  cannot  be  accounted  for  by  the  limited  information 
given  by  such  tests,  have  forced  investigators  to  probe  more  deeply 
into  the  complicated  kinematic  forces  which  seem  to  have  such  a 
great  influence  upon  the  "  life  "  of  the  metal.  It  is  now  a  well- 
known  fact  that,  if  a  stress  is  applied  a  great  number  of  times,  i.e., 
repeated,  each  application  being  made  before  the  material  has  had 
time  to  recover  from  the  preceding  stress,  the  material  will  event- 
ually break  even  though  the  stress  is  below  the  elastic  limit  of  the 
material.  These  repeated  stresses  upon  steel  cause  a  gradual  dis- 
turbance of  the  structure  and  its  component  particles,  which  greatly 
weakens  the  material,  and  is  called  fatigue.  The  resistance  to 
fatigue  and  its  numerical  test  value  may  be  termed  the  endurance 
of  the  steel.  The  stresses  embodied  under  the  heading  of  fatigue 
may  be  broadly  classed  as  repeated  static  stresses  and  acceleration 
stresses  ranging  from  zero  to  maximum  or  from  a  negative  maximum 
to  a  positive  maximum — alternating— stresses. 

Fatigue  Stresses. — These  stresses  are  produced  in  a  machine 
part  by  an  external  force  or  forces  of  varying  strength  and  direction 
acting  upon  the  part.  When  the  force  is  produced  by  a  continu- 
ously varying  acceleration  or  retardation  of  masses  taking  part  in 
the  movement  of  the  machine  part,  they  may  be  conveniently  termed 
acceleration  stresses.1  Typical  stresses  of  this  category  are  the 
revolving  shaft  stress  on  a  loaded  wheel  or  machine  axle,  the  piston 
pressure  and  the  acceleration  pressure  of  the  movable  parts  in  the 
piston  rod  and  crank-shaft  of  high-speed  steam  and  oil  engines. 
These  perpetual  stresses  or  so-called  fatigue  stresses  are  the  essential 
ones'  in  the  movable  parts  of  most  high-speed  machines,  and  a  knowl- 
edge of  the  capacity  of  the  material  to  resist  them  should  serve  as 
a  basis  for  the  selection  of  the  material  and  design. 

Rotary  Bending. — Such  static  endurance  tests  may  be  carried 
out  in  a  machine  of  the  rotary  bending  type,  such  as  the  Wohler 
or  the  White-Souther  machines.  From  a  study  of  a  large  number  of 
1  J.  O.  Roos  af  Hjelmsaeter,  Int.  Assoc.  Test.  Mat.,  1912,  Vol.  II,  No.  9. 


THE   TESTING  OF  STEEL  7 

experiments  made  on  a  rotary  bending  machine  of  the  Wohler 
design,  Foos  concludes  that  such  endurance  tests  are  not  suitable  as 
specification  tests,  but  are  of  great  value  in  the  selection  of  material 
and  the  heat  treatment  for  various  purposes. 

On  the  other  hand,  the  real  value  of  the  rotary  bending  test  as  a 
criterion  of  the  brittleness-fatigue  endurance  has  been  of  late  greatly 
questioned.  That  the  results  usually  obtained  are  largely  indicative 
of  the  elastic  limit  alone  is  probably  more  in  accord  with  our  present- 
day  knowledge.  The  results  from  a  series  of  tests  conducted  by 
Foos  upon  a  Wohler  type  rotary  bending  machine  with  steels  of 
0.11,  0.40  and  0.65  per  cent,  carbon,  given  in  the  following  table, 
would  tend  to  support  the  latter  theory,  as  one  would  naturally 
expect  from  past  experience  that  the  0.40  per  cent,  carbon  steel  would 
have  a  greater  fatigue-resisting  strength  than  the  0.65  carbon  steel. 


ROTARY  BENDING  TESTS,  WOHLER  MACHINE 


Chemical. 

Static  Properties. 

Endurance 
Limit. 

Fiber 

Stress 

d 

o> 

ta 

fl 

o 

,0 

<p 
1 

0 

,c 

1 

c 
o 

Tensile 
Strength 
Lbs.  per 
Sq.  In. 

Elastic 
Limit 
Lbs.  per 
Sq.  In. 

Elonga- 
tion, 
Per  Cent 
in 
3.94  Ins. 

in  Kg. 
giving 
I  racture 
after 
1  Million 

I 

£ 

s 

c3 

j2 

•H 

Revolu- 

OQ 

EH 

u 

s 

fe 

cc 

CO 

tions. 

R 

A 

0.11 

0.33 

0.019 

0.013 

0.01 

49,770 

32,990 

34.1 

16 

Si 

A 

0.40 

0.51 

0.027lo.011 

0.20 

82,760 

50,770 

23.8 

22 

S2 

O.T. 

0.40 

0.51 

0.027 

0.011 

0.20 

109,780 

70,820 

15.6 

28 

Ti 

A 

0.65 

0.49 

0.023 

0.007 

0.20 

116,040 

50,900 

14.3 

25 

T2 

O.T. 

0.65 

0.49 

0.023 

0.007 

0.20 

151,020 

94,560 

11.0 

38 

Treatment.     "  A,"  heated  at  1560°  F.  for  30  minutes,  and  air-cooled. 

"  O.T.,"  heated  at  1560°  F.  for  30  minutes,  quenched  in  mineral  oil,  and 
re-heated  to  1025°  F. 

Suddenly  Applied  Loads. — Machine  parts  at  one  time  or  another 
may  be  exposed  to  abnormal  working  loads  such  as  may  result  from 
a  single  accidental  blow  or  a  sudden  retardation  of  masses  in  motion, 
and  which  in  any  case  cannot  be  supposed  to  be  frequently  repeated. 
Such  abnormal  stresses  are  therefore  in  the  nature  of  suddenly  applied 
loads  or  impact  stresses,  and  constitute  a  different  group  from  those 
previously  discussed  under  static  stresses.  It  is  evident  that  such 
.stresses  demand  that  the  material  be  able  to  sustain  a  great  work  of 
deformation  for  a  single  or  a  few  impacts  without  rupture — that  is, 


8  STEEL  AND   ITS  HEAT  TREATMENT 

the  machine  part  shall  sustain  as  little  damage  or  deformation  as 
possible.  In  general  the  "  ductility  "  (elongation  and  reduction  of 
area)  has  for  a  long  time  been  used  as  a  measure  of  the  work  of  rup- 
ture. But,  although  such  tests  are  of  comparative  value,  they  do 
not  measure  either  the  ductility  under  impact  or  the  strength  or 
resistance  under  impact. 

Drop  Test. — The  drop  test  as  commercially  applied  may  be 
described  as  an  aggravated  bend  test  on  a  large  scale.  It  is  a  relative 
or  qualitative  test  only,  usually  made  on  a  full-size  forging,  to  deter- 
mine roughly  the  homogeneity  of  the  metal  and  its  ductility  under 
shock.  We  have  arbitrarily  separated  this  test  from  the  "  impact  " 
tests,  reserving  the  latter  as  applied  to  the  specific  measurement  of 
the  impact  strength  upon  a  more  or  less  standardized  test  bar. 
The  most  common  application  of  the  drop  test  is  that  of  locomotive 
axles,  in  which  it  is  required  that  the  axle  shall  stand  a  specified, 
number  of  blows  at  a  given  height  without  rupture  and  without 
exceeding,  as  a  result  of  the  first  blow,  a  certain  deflection. 

Impact  Strength. — The  impact  test  is  used  to  determine  the 
ability  of  the  metal  to  withstand  a  suddenly  applied  load  in  the 
nature  of  an  impact  or  shock,  thus  detecting  brittleness  or  lack  of 
toughness.  This  function  is  called  resilience  by  the  foreign  technical 
world,  referring  to  fragility  or  the  converse  of  brittleness,  and  is 
stated  in  terms  of  the  specific  work  of  rupture  under  impact.  It 
should  not  be  confused  with  the  English  word  " resilience,"  which 
is  interpreted  in  this  country  as  "  springiness."  This  fragility  is 
not  defined  by  the  tensile  test,  although  an  experienced  steel  man, 
from  an  examination  of  the  size  and  aspect  of  the  grain  and  other 
conditions  of  the  fracture  of  the  test  piece,  can  usually  express  an 
opinion  as  to  the  fragility,  but  he  cannot  assess  any  definite  value. 
Although  the  drop  test  specifies  the  fragility  in  a  qualitative  manner, 
it  does  not  measure  the  actual  resistance  to  rupture,  and  is  therefore 
but  an  imperfect  test.  In  order  to  overcome  such  objections  and 
to  arrive  at  a  definite  value,  machines  have  been  devised  which 
break  a  special  notched  bar  by  a  blow — the  force  required  to  rupture 
the  metal  being  measured  in  kilogram-meters  or  foot-pounds. 
Notched  test  bars  are  used  in  order  to  localize  the  deformations. 
The  blow  must  be  delivered  with  sufficient  velocity  to  bring  out  the 
desired  brittleness  functions.  This  blow  or  impact  may  be  obtained 
by  a  falling  weight  (the  Fremont  machine),  by  a  falling  pendulum 
(the  Charpy  principle),  or  by  a  revolving  fly-wheel  bearing  a  releas- 
able  knife  (the  Guillery  machine),  these  three  representing  the  most 


THE   TESTING  OF  STEEL  9 

common  types  of  impact  machines  in  use  abroad,  as  well  as  the 
Olsen  pendulum  type  (using  a  test  specimen  in  the  form  of  a  canti- 
lever) in  this  country. 

Impact  Tests. — Fremont *  recommends  as  the  ideal  conditions 
to  be  realized  in  the  application  of  the  impact  test:  (1)  A  minimum 
drop  of  four  meters,  or  proportional  to  the  impact  speed;  (2)  the 
weight  of  the  anvil  block  to  be  equal  to  at  least  forty  times  the 
weight  of  the  tup;  (3)  sufficient  ease  and  rapidity  of  adjustment  of 
the  machine. 

On  account  of  the  many  factors  entering  into  the  problem  and 
the  numerous  designs  of  impact  testing  machines,  the  majority  of 
the  testing  associations  have  abstained  from  prescribing  any  special 
type  of  apparatus  for  performing  the  test.  The  Copenhagen  Con- 
gress (1909)  of  the  International  Society  for  Testing  Materials  has, 
however,  recommended  the  use  of  a  standard  notched  test  bar  of 
30X30X160  mm.,  or  a  smaller  bar  of  10X10  mm.  cross-section 
when  the  larger  size  is  not  available.  On  the  contrary,  there  are 
many  who  believe  that  a  smaller,  rectangular  test  bar  reveals  more 
clearly  the  local  defects  which  form  the  nucleus  of  future  cracks,  etc. 

Use  of  Impact  Tests.— The  fragility  test  has  a  double  purpose — 
to  point  out  steel  which  is  defective,  either  inherently  or  by  incorrect 
heat  treatment;  and  to  act  as  a  valuable  aid  for  the  adjustment  of 
a  proper  heat  treatment.  It  is  evident  that  steels  burdened  with 
sulphur  and  phosphorus,  or  rotten  with  piping  and  segregation,  will 
always  remain  brittle  whatever  one  may  do.  But  ordinary,  sound 
stock,  properly  treated,  is  nearly  always  non-brittle.  The  degree 
of  brittleness  will  of  course  vary  according  to  the  composition, 
treatment  and  use  of  the  different  steels.  The  effect  of  heat  treat- 
ment upon  the  impact  strength  is  very  great,  so  that  due  care  should 
be  used  in  so  adjusting  the  chemical  composition  and  treatment  of 
the  steel  as  to  give  the  best  combination  for  the  work  in  hand. 
The  impact  test,  in  conjunction  with  other  tests,  gives  a  quick 
method  of  determining  a  quality  the  importance  of  which  is  yearly 
becoming  more  prominent  scientifically;  commercially,  however, 
the  impact  tests  are  so  unreliable,  or  vary  so  greatly,  that  they  can 
hardly  be  used  with  any  degree  of  accuracy. 

Fatigue  Impacts. — An  impact  or  shock  has  a  considerably  greater 
effect  than  a  stress  slowly  applied,  and  if  repeated  a  sufficient  num- 
ber of  times  will  eventually  result  in  the  rupture  of  the  specimen. 
When  such  frequently  repeated  stresses  are  comparatively  small — 
1  Ch.  Fremont,  Proc.  Int.  Assoc.  Test.  Mat,,  Vol.  II,  No.  9,  1912. 


10  STEEL  AXD   ITS   HEAT  TREATMENT 

that  is,  are  well  below  the  elastic  limit  of  the  material — they  may  be 
termed  fatigue  impacts.  Their  measurement,  as  determined  by  the 
energy  or  amount  of  work  they  represent,  is  a  principal  component 
of  the  dynamic  strength  of  the  material.  Apparatus  for  thus  test- 
ing the  material  is  developed  upon  the  principle  of  alternating 
impacts.  As  practical  examples  of  such  stresses  in  commercial 
application  there  may  be  mentioned  the  stresses  to  which  the  axles 
of  locomotives  or  railway  cars  are  subjected  every  time  the  wheel 
passes  over  a  rail  joint,  or  the  impact  stresses  sustained  by  different 
parts  of  a  motor  car  when  passing  over  bad  roads,  or  in  changing  to 
different  speeds,  etc. 

Alternating  Impact  Machines. — Various  machines  for  establishing 
comparative  numerical  values  for  this  dynamic  strength  have  been 
designed,  in  the  endeavor  to  produce  an  alternating  flexure  and  at 
the  same  time  deliver  a  blow  or  impact.  This  has  been  accomplished 
by  applying  blows  to  the  upper  part  of  a  test  piece  with  the  aid  of 
two  pendulum  balls  which  are  made  to  fall  alternately  from  opposite 
directions  from  a  certain  height  against  the  test  piece.  Other 
machines  have  been  patterned  along  the  lines  of  the  Upton-Lewis 
machine,  keeping  the  fiber  stress  well  within  the  elastic  limit. 

Alternating  Impact  Test  Results. — The  study  of  a  great  number  of 
alternating  impact  tests  of  a  comparative  nature,  the  views  of  other 
engineers,  and  the  study  of  steel  parts  broken  in  service,  would  lead 
the  author  to  the  opinion  that  the  dynamic  strength  (using  the  term 
in  its  broadest  meaning)  of  straight  carbon  steels  reaches  a  maxi- 
mum at  0.25  to  0.35  per  cent,  carbon,  with  perhaps  even  narrower 
limits  of  0.25  to  0.30  per  cent,  carbon.  Further,  the  maximum 
endurance  is  obtained  when  the  steels  have  been  properly  hardened 
from  a  temperature  slightly  over  the  upper  critical  range  and  tough- 
ened at  a  temperature  of  1200°  to  1250°  F.  The  maximum  com- 
bination of  static  working  strength,  ductility,  resistance  to  shock 
and  vibration,  and  endurance  probably  is  obtained  in  straight  car- 
bon steels  with  0.35  to  0.45  per  cent,  carbon  when  subjected  to  the 
above  treatment. 

Relation  of  Various  Tests. — There  does  not  seem  to  be  any  simple 
relation  between  the  elastic  limit  at  steady  tensile  stress  and  the 
limit  of  endurance  in  the  rotary  bending  test,  although  some  engineers 
consider  it  as  a  "  reflection  of  the  elastic  limit."  The  rotary  bending 
is  undoubtedly  less  than  the  former,  and  according  to  the  investiga- 
tions of  some  engineers,  the  limit  of  endurance  (rotary  bending)  will 
generally  amount  to  about  50  to  80  per  cent,  of  the  elastic  limit. 


THE   TESTING   OF  STEEL  11 

According  to  experiments  made  by  Foos  on  straight  carbon  steels, 
the  limit  of  endurance  for  rotary  bending  and  alternating  impact 
within  the  elastic  limit  agree  fairly  closely. 

A  high  value  in  the  work  of  rupture  in  the  impact  test  may  be 
considered  to  give  comparative  security  in  the  case  of  occasional 
abnormal  over-loads. 

Thus,  the  requirements  for  a  high-quality  steel  for  machine  parts 
are  a  high  limit  of  endurance  for  the  normal  stresses  and  a  high  figure 
of  rupture  for  the  abnormal  stresses.  As  a  rule,  these  qualities  are 
opposed  to  each  other  in  ordinary  materials,  and  it  must  rest  upon 
experience  as  to  which  to  give  the  preference;  in  parts  which  suffer 
through  vibration  and  other  fatigue  stresses,  it  will  probably  be 
wiser  to  give  preference  to  the  endurance  properties.  It  must  be 
remembered  that  heat  treatment  and  the  various  alloys  may 
entirely  change  the  different  properties. 

Hardness. — "  Hardness l  may  be  defined  as  the  property  of 
resisting  penetration,  and  conversely,  a  hard  body  is  one  which, 
under  suitable  conditions,  readily  penetrates  a  softer  material. 
There  are,  however,  in  metals  various  kinds  or  manifestations  of 
hardness  according  to  the  form  of  stress  to  which  the  metal  may  be 
subjected.  These  include  tensile  hardness,  cutting  hardness,  abra- 
sion hardness,  and  elastic  hardness;  doubtless  other  varieties  could 
also  be  recognized  when  the  experimental  conditions  are  modified 
so  as  to  bring  into  operation  properties  of  the  material  in  addition 
to  that  of  simple,  or  what  may  be  conveniently  called  mineralogical 
hardness.  This  has  been  defined  by  Dana  as  '  the  resistance  offered 
by  a  smooth  surface  to  abrasion.'  The  usual  quantitative  tests  for 
hardness  are  static  in  character,  but  the  conditions  are  profoundly 
modified  when  the  penetrating  body  is  moving  with  greater  or  less 
velocity.  The  resistance  to  the  action  of  running  water;  to  the 
effect  of  a  sandblast;  or  to  the  result  of  the  pounding  of  a  heavy 
locomotive  on  a  steel  rail,  afford  examples  of  what  might  perhaps  for 
purposes  of  distinction  be  called  dynamic  hardness,  which  is  a 
branch  of  the  subject  which  has  received  little  quantitative 
examination." 

Brinell  Hardness.— The  Brinell  test  consists  in  the  pressing  of  a 
hardened  steel  ball  into  the  surface  of  the  object  under  test  by 
means  of  a  fixed  load.  The  dimensions  of  the  impression  thus  ob- 
tained form  the  basis  for  calculating  the  hardness.  If  the  number 
of  kilograms  making  up  the  load  is  divided  by  the  spherical  surface 
1  Thomas  Turner,  Inst.  Journ.,  May,  1909. 


12  STEEL  AND   ITS   HEAT   TREATMENT 

of  the  impression,  expressed  in  square  millimeters,  a  number  is 
obtained,  expressing  the  pressure  exerted  per  square  millimeter  of 
ball  impression.  This  number  is  now  accepted  as  a  measure  of 
hardness,  and  it  is  hence  called  the  Brinell  hardness  number.  It  is 
generally  sufficient  to  utilize  the  diameter  of  the  ball  impression 
itself  as  a  measure  of  the  hardness.  In  order  to  make  tests  exo- 
cuted  at  different  works  directly  comparable,  a  standard  ball  of  10 
mm.  and  a  load  of  3000  kilograms  are  used. 

Brinell  Transference  Number. — It  is  a  well-established  fact  that 
the  Brinell  hardness  numbers  follow  very  closely  the  tensile  strength 
of  the  same  types  of  steel,  whether  the  steel  be  "  in  the  natural," 
or  whether  it  has  been  subjected  to  some  heat-treatment  process. 
For  this  reason  it  is  particularly  applicable  to  the  rapid  testing  of 
steel  from  which  it  would  be  impracticable  to  take  regular  tensile 
tests.  A  few  comparisons  between  the  actual  tensile  strength  as 
obtained  by  pulling  tests  and  the  hardness  number  obtained  from 
the  test  pieces  will  serve  as  a  basis  for  future  calculations.  By  ob- 
taining such  a  "  transference  number  "  the  probable  tensile  strength 
of  the  steel  in  question  may  be  easily  computed  by  multiplying  the 
hardness  number  by  the  "  transference  "  number.  This  transference 
number  will  vary  with  the  chemical  composition  of  the  steel,  and 
to  a  small  extent  with  the  manner  of  testing  (whether  with  or  across 
the  grain)  and  between  tempered  and  annealed  steels.  On  the  whole, 
however,  the  test  is  comparatively  accurate  for  steels  purchased 
or  made  under  the  same  general  chemical  specification.  For  straight 
carbon  steels  this  transference  number  may  be  said  to  be  about 
500  to  520.  The  Brinell  method  has  a  great  advantage  over  the 
scleroscope  in  that  it  does  not  require  an  extremely  smooth  or 
polished .  surf  ace  for  the  test;  the  removal  of  scale  by  filing  is  prac- 
tically the  only  requirement. 

Many  companies  are  standardizing  their  heat  treatment  product 
by  taking  the  hardness  of  each  piece  treated,  thus  ensuring  a  close 
range  of  the  desired  tensile  properties.  On  account  of  the  influence 
of  the  size  of  the  original  section  upon  the  physical  results  as  obtained 
by  the  pull-test,  it  is  much  easier  to  determine  the  transference 
number  for  the  specific  grade  of  steel  being  treated,  and  then  vary 
the  toughening  temperature  so  as  to  give  the  desired  Brinell  hardness 
number.  It  is  the  author's  experience  that  this  method  is  fairly 
accurate,  and  that  the  Brinell  number  will  give  a  close  approximation 
of  the  true  tensile  strength  regardless  of  whether  the  treated  bar  is 
1  in.  or  5  ins.  thick.  The  Brinell  method  is  simple  and  com- 


THE   TESTING  OF   STEEL 


13 


mercially  efficient,  with  the  exception  of  either  very  thin  or  highly 
tempered  material. 

For  reference  convenience,  the  relation  between  the  diameter  of 
the  impression  and  the  hardness  number  is  given  in  the  following 
table: 

BRINELL'S   HARDNESS-NUMBERS 
Diameter  of  Steel  Ball  =  10  mm. 


Diameter 
of  Ball 
Impression 
mm. 

h 

O 

"°& 

ll-§« 

•o  3^§ 

J§£  (SCO 

B 

Diameter 
of  Ball 
Impression 
mm. 

Hardness 
Number  for 
a  Load  of 
3000  Kgr. 

Diameter 
of  Ball 
Impression 
mm. 

§ 

.*l& 

rcXi^HH 
§  S  00 
fg*l 

S^  cdec 

Diameter 
of  Ball 
Impression 
mm. 

Hardness 
Number  for 
a  Load  of 
3000  Kgr. 

Diameter 
of  Ball 
Impression 
mm. 

Hardness 
Number  for 
a  Load  of 
3000  Kgr. 

2 

946 

3 

418 

4 

228 

5 

143 

6 

95 

2.05 

898 

3.05 

402 

4.05 

223 

5.05 

140 

6.05 

94 

2.10 

857 

3.10 

387 

4.10 

217 

5.10 

137 

6.10 

92 

2.15 

817 

3.15 

375 

4.15 

212 

5.15 

134 

6.15 

90 

2.20 

782 

3.20 

364 

4.20 

207 

5.20 

131 

6.20 

89 

2.25 

744 

3.25 

351 

4.25 

202 

5.25 

128 

6.25 

87 

2.30 

713 

3.30 

340 

4.30 

196 

5.30 

126 

6.30 

86 

2.35 

683 

3.35 

332 

4.35 

192 

5.35 

124 

6.35 

84 

2.40 

652 

3.40 

321 

4.40 

187 

5.40 

121 

6.40 

82 

2.45 

627 

3.45 

311 

4.45 

183 

5.45 

118 

6.45 

81 

2.50 

600 

3.50 

302 

4.50 

179 

5.50 

116 

6.50 

80 

2.55 

578 

3.55 

293 

4.55 

174 

5.55 

114 

6.55 

79 

2.60 

555 

3.60 

286 

4.60 

170 

5.60 

112 

6.60 

77 

2.65 

532 

3.65 

277 

4.65 

166 

5.65 

109 

6.65 

76 

2.70 

512 

3.70 

269 

4.70 

163 

5.70 

107 

6.70 

74 

2.75 

495 

3.75 

262 

4.75 

159 

5.75 

105 

6.75 

73 

2.80 

477 

3.80 

255 

4.80 

156 

5.80 

103 

6.80 

71.5 

2.85 

460 

3.85 

248 

4.85 

153 

5.85 

101 

6.85 

70 

2.90 

444 

3.90 

241 

4.90 

149 

5.90 

99 

6.90 

69 

2.95 

430 

3.95 

235 

4.95 

146 

5.95 

97 

6.95 

68 

Shore  Scleroscope. — The  principle  of  the  Shore  scleroscope  hard- 
ness test  is  based  upon  the  height  of  rebound  of  a  diamond-faced  ham- 
mer when  dropped  from  a  standard  height  upon  the  surface  of  the 
material  to  be  tested.  One  of  the  great  disadvantages  of  the  sclero- 
scope, in  the  author's  experience,  is  that  it  requires  a  highly  polished 
surface  in  order  to  obtain  anywhere  near  accurate  and  comparative 
results.  The  scleroscope  readings  are  probably  more  indicative  of 
the  elastic  limit  than  of  the  tensile  strength.  The  following  experi- 
ments made  with  the  scleroscope,  illustrating  the  results  to  be  ob- 
tained with  different  methods  of  polishing,  may  afford  some  explana- 
tion of  the  variations  often  characteristic  of  this  instrument: 


14 


STEEL  AND   ITS   HEAT  TREATMENT 


Specimen  rough  filed readings,  25  to  30 

smooth  filed readings,  27  to  32 

rubbed  with  emery  cloth  No.  1  readings,  32 
rubbed  with  emery  paper  No.  00  readings,  32 
polished  with  diamantine readings,  33 

Ballistic  Test. — The  ballistic  test  is  distinct  from  the  static  hard- 
ness tests  above  described  in  that  it  is  a  measure  of  the  dynamic 
hardness  by  resistance  to  penetration  under  violent  impact.  From 
the  author's  experience  with  protective-deck  and  bullet-proof  steel, 
the  Brinell  hardness  is  only  a  measure  of  the  tensile  strength  and  does 
not  give  a  comprehensive  idea  of  the  ballistic  qualities  of  the  plate  or 
sheet.  Similarly,  tests  made  by  the  Italian  Government  show  that 
none  of  the  tests  mentioned  in  this  chapter,  either  static  or  dynamic, 
nor  their  ensemble,  gives  a  sufficient  indication  of  the  resistance 
which  such  plates  will  oppose  in  firing  tests. 

Wear.— Wear,  or  the  hardness  of  material  as  indicated  by  its 
resistance  to  abrasive  action,  has  demanded  considerable  attention 
of  late  on  account  of  the  increased  development  of  high-power 
machines  and  engines.  The  increased  weight  put  upon  locomotive 
axles  and  rails,  the  higher  speeding  of  rotating  parts,  and  a  similar 
tendency  to  wear  in  other  machine  parts  have  all  necessitated 
further  study  of  this  important  property  of  steel. 

For  resistance  to  abrasion,  Robin l  has  arrived  at  the  following 
values,  these  being  obtained  upon  annealed  steel  with  carbon  con- 
tents as  given,  manganese — 0.25  per  cent,  to  0.30  per  cent.;  phos- 
phorus— 0.015  per  cent,  to  0.40  per  cent.;  silicon — about  0.20  per 
cent. : 

WEAR    BY   ABRASION.     ANNEALED   STEEL 


Carbon 
Content 
Per  Cent. 

Abrasive 
Figure. 

Carbon 
Content 
Per  Cent. 

Abrasive 
Figure. 

0.07 

295 

0.65 

308 

0.12 

293 

0.69 

280 

0.25 

312 

0.83 

258 

0.38 

350 

1.00 

252 

0.60 

312 

1.03 

252 

J.  Robin,  Inst.  Journ.,  II,  1910. 


THE  TESTING  OF  STEEL  15 

This  would  tend  to  show  that  the  wear  is  not  proportional  to  the 
carbon  content  in  annealed  carbon  steels,  and  that  the  maximum 
wear  might  be  expected  with  steels  of  approximately  0.40  per  cent, 
carbon.  He  further  concludes  from  other  experiments  that  the 
abrasive  wear  increases  with  the  percentage  of  phosphorus  and 
diminishes  with  the  amount  of  manganese  and  silicon. 


CHAPTER  .II 
THE  STRUCTURE   OF   STEEL 

Steel. — Steel  is  an  alloy,  the  principal  and  essential  chemical 
constituents  of  which  are  iron  and  carbon.  With  these  there  are 
usually  certain  impurities,  such  as  phosphorus,  sulphur,  and  silicon, 
which  have  not  been  entirely  eliminated  during  the  process  of 
manufacture,  as  well  as  manganese — and  perhaps  other  alloys  such 
as  nickel  and  chrome — which  may  have  been  intentionally  added 
for  a  definite  purpose.  Of  the  elements  which  go  to  make  up  ordinary 
steel,  the  manganese,  phosphorus,  sulphur  and  silicon — the  impurities 
— generally  total  to  about  1  per  cent. ;  the  carbon  will  vary  from  a 
few  hundredths  of  1  per  cent,  to  about  2  per  cent.;  and  the 
balance  will  be  iron. 

Furthermore,  steel  is  not  a  simple  substance  like  copper  or  gold, 
but  isjnore  like  granite,1  in  that  it  is  made  up  of  a  number  of  individual 
grains  (let  us  say)  or  "  minerals,"  corresponding  to  the  quartz,  mica 
and  feldspar  of  the  granite.  Thus,  in  steel  which  has  cooled  slowly 
from  a  high  temperature,  we  have  "  ferrite,"  "  cementite  "  and 
"  pearlite."  And  just  as  the  relative  amounts  of  the  quartz,  mica 
and  feldspar  may  vary  in  the  rocks  of  the  granite  class,  so  will  the 
relative  proportions  of  ferrite,  cementite  and  pearlite  vary  in  dif- 
ferent steels  according  to  the  specific  chemical  composition  of  the 
steel  as  a  whole. 

Cementite. — As  we  have  stated  above,  the  carbon  and  iron  are 
the  essential,  as  well  as  controlling,  elements  in  the  steel — and  this 
is  particularly  true  of  the  carbon.  In  steels  which  have  cooled  slowly 
from  a  high  temperature,  the  carbon  is  first  and  always  combined 
with  a  ^definite  amount  of  iron  to  form  a  "  carbide  of  iron,"  corre- 
sponding to  the  chemical  symbol  FesC.  This  compound  consists  of 
6.6  per  cent,  carbon  and  93.4  per  cent,  iron,  and  is  micrographically 
known  as  "  cementite."  The  balance  of  the  iron,  is  practically 
carbon-free  and  is  known  as  "  ferrite." 

Pearlite. — Now  during  the  process  of  cooling  at  a  moderate  rate 
from  a  red  heat,  this  cementite  will  form  a  mechanical  mixture  with  a 

16 


THE   STRUCTURE   OF   STEEL  17 

definite  amount  of  ferrite,  so  that  the  resultant  will  contain  approx- 
imately 0.9  per  cent,  carbon.  This  new  constituent  is  called  "  Pearl- 
ite  "  and  usually  consists  of  interst ratified  layers  or  bands  of.  ferrite 
and  cementite.  Pearlite  is  regarded  as  a  separate  and  distinct 
constituent  of  steel,  as  it  forms  distinct  "  grains  "  when  present  in 
any  appreciable  quantity,  always  contains  this  definite  percentage 
of  carbon,  and — as  will  be  explained  later — is  always  born  at  a  definite 
range  of  temperatures. 

Eutectoid  Steels. — From  this  it  will  be  seen  that  a  steel  contain- 
ing 0.9  per  cent,  carbon  will  consist  entirely  of  pearlite.  Such  steels 
are  known  as  "  eutectoid  "  steels,  and  that  ratio  of -carbon  as  the 
eutectoid  ratio. 

Hypo-eutectoid  Steels. — Steels  containing  less  than  this  eutectoid 
ratio  of  carbon  will  consist  of  a  definite  amount  of  pearlite,  varying 
according  to  the  carbon  content  of  the  steel  proper,  and  the  balance 
in  "  free  "  or  "  excess  "  ferrite.  These  steels  are  called  "  hypo- 
eutectoid  "  steels,  as  they  contain  less  than  0.9  per  cent,  carbon. 

Hyper-eutectoid  Steels. — Similarly,  if  the  carbon  content  exceeds 
0.9  per  cent,  carbon,  there  will  not  be  sufficient  ferrite  to  inter- 
stratify  with  all  of  the  cementite,  so  that  these  steels  will  consist  of 
pearlite  plus  free  cementite.  Such  steels  are  called  "  hyper-eutec- 
toid  "  steels. 

Expressing  this  in  a  different  way,  we  may  say  that  very  low 
carbon  steels  are  made  up  of  ferrite  with  a  little  pearlite.  With 
increase  in  the  carbon  content  of  the  steel,  the  amount  of  pearlite 
will  likewise  increase,  with  a  corresponding  diminution  in  the  amount 
of  free  ferrite,  until  at  0.9  per  cent,  carbon  the  steel  will  be  wholly 
pearlitic.  Beyond  this  point  the  amount  of  pearlite  will  decrease, 
with  a  corresponding  increase  in  the  amount  of  free  cementite. 

Structure  of  Slowly  Cooled  Steels. — In  slowly  cooled  steels  we 
may  therefore  tell  with  great  accuracy  the  approximate  structural 
composition  of  the  steel.  And,  vice  versa,  knowing  the  relative  pro- 
portions of  pearlite  and  ferrite  or  cementite,  as  determined  micro- 
scopically, we  may  determine  the  approximate  carbon  content  of  the 
steel. 

This  is  represented  graphically  in  Sauveur's  diagram  as  shown  in 
Fig.  1,  in  which  the  percentage  carbon  is  represented  by  the  ab- 
scissae and  the  percentage  constituents  by  the  ordinates. 

These  .facts  are  also  illustrated  by  the  photomicrographs  in  Figs. 
2  to  9,  representing  the  structure  of  slowly  cooled  steel  of  0.06,  0.18, 
0.32,  0.49,  0.57,  0.71,  0.83  and  1.46  per  cent,  carbon  respectively.  In 


18 


STEEL  AND   ITS   HEAT   TREATMENT 


Figs.  3  to  7  it  will  be  seen  that  the  pearlite  (dark;  structure  not 
brought  out  by  the  etching)  gradually  increases  in  amount,  while  the 
ferrite  (light)  proportionally  diminishes.  Fig.  8  shows  a  steel  of  the 
eutectoid  composition  in  which  the  ferrite  (dark)  and  the  cementite 
(light)  are  interstratified  with  each  other,  there  being  practically  no 
"  free  "  ferrite  such  as  characterized  the  lower  carbon  steels.  Fig.  9 
shows  the  structure  of  a  1.46  per  cent,  carbon  steel  in  which  the  free 
cementite  (white)  occurs  as  a  network  between  the  grains  of  pearlite 
(dark).  In  Fig.  10,  showing  a  steel  of  just  above  the  eutectoid 


KX) 


80 


40 


20 


Ferrite 

/ 

7T; 

^~^OM 

uentite 

>•>*: 

/ 

/ 

/ 

/ 

Pea 

rlite 

/ 

/ 

~/ 

0.2                 0.4                 0.6                 0.8                 1.0                 1,2                 1.' 
Per  Cent  Carbon 

FIG.  1. — Ferrite-pearlite-cementite  Diagram.     (Sauveur.) 


ratio,  we  see  the  first  appearance  of  the  free  cementite  between  the 
pearlite  crystals.  In  Fig.  11  we  have  this  whole  range  represented 
by  means  of  case  carburizing  a  "  dead  soft  "  steel. 

Physical  Properties  Dependent  upon  Constituents. — Upon  the 
relative  proportions  of  these  constituents  will  depend  the  physical 
properties  of  the  slowly  cooled  steel,  neglecting  for  the  time  being 
their  relative  arrangement.  Each  of  these  components — ferrite, 
pearlite  and  cementite — has  certain  physical  characteristics  with 
which  we  must  be  familiar  in  order  to  gain  some  idea  of  the  proper- 
ties of  such  steels. 


THE   STRUCTURE  OF  STEEL 


19 


FIG.  2. — 0.06  per  cent.  Carbon.     Approximately  Pure  Ferrite.     X75.     (Ord- 
nance Dept.) 


v    >     *-'V 

i      -  .V 


FIG.  3.— 0.18  per  cent,  Carbon.     Ferrite   (White)  and  Pearlite  (Dark).     X75. 

(Ordnance  Dept.) 


20 


STEEL  AND  ITS  HEAT  TREATMENT 


IG.  4.— 0.32  per  cent.  Carbon.     Ferrite  (White)  and  Pearlite  (Dark). 

(Ordnance  Dept.) 


X75. 


JPIG  5, — 0.49  per  cent.  Carbon.     Ferrite  (White)  and  Pearlite  (Dark).     X75. 

(Ordnance  Dept.) 


THE  STRUCTURE  OF  STEEL 


21 


0.57  per  cent.  Carbon.     Ferrite  and  Pearlite.     X75.     (Ordnance  Dept.) 


• 


^ 

M 


FIG.  7. — 0.71  per  cent.  Carbon.     Ferrite  and  Pearlite.      X75.     (Ordnance  Dept.) 


22 


STEEL   AND   ITS   HEAT   TREATMENT 


FIG.  8.— 0.83  per  cent.  Carbon.     Pearlite.     X485.     (Ordnance  Dept.) 


FIG.  9. — 1.46  per  cent.  Carbon. 


Pearlite  and  Cementite  (White), 
nance  Dept.) 


X75.     (Ord- 


THE   STRUCTURE   OF   STEEL  23 

Ferrite.— Ferrite  is  soft,  ductile  and  relatively  weak.  It  has  a 
Tensile  strength  of  approximately  40,000  to  50,000  Ibs.  per  square 
inch,  with  an  elongation  of  about  40  per  cent,  in  2  ins.  Ferrite 
in  itself  has  no  hardening  power  as  applicable  to  industrial  purposes. 
It  is  magnetic  and  has  a  high  electric  conductivity.  Its  appear- 
ance under  the  microscope  has  been  shown  in  the  photomicro- 
graphs previously  mentioned — that  is,  as  polyhedral  crystals  in  the 
low  carbon  steels. 

Pearlite. — As  previously  mentioned,  the  common  occurrence  of 
pearlite  in  slowly  cooled  steels  is  in  the  lamellar  formation,  is  being 


FIG.  10. — Laminated  Pearlite  and  First  Appearance  (as  Veins  between  Grains) 
of  the  Excess  Cementite.      XlOO.     (Titanium  Alloys  Mfg.  Co.) 

composed  of  alternate  plates  of  ferrite  (showing  dark  under  the 
microscope)  and  cementite  (showing  white  under  the  microscope). 
As  will  be  shown  later,  under  different  rates  of  cooling  pearlite  may 
exist  in  other  formations  and  dependent  upon  the  relative  arrange- 
ment of  the  ferrite  and  cementite  of  which  it  is  composed;  some 
of  these  various  modifications  are  shown  in  Figs.  8  and  10.  Normal 
pearlite,  that  is,  interstratified  bands  of  ferrite  and  cementite  such 
as  shown  in  Fig.  8,  has  a  tensile  strength  of  approximately  125,000  to 
130,000  Ibs.  per  square  inch,  with  an  elongation  of  about  10  per  cent, 
in  2  ins. 


24 


STEEL  AND   ITS   HEAT  TREATMENT 


Cementite. — The  properties  of  cementite  are  very  little  known 
with  the  exception  of  its  great  hardness  and  brittleness,  which  are  a 
maximum.  Free  cementite,  that  is,  unassociated  with  ferrite  to 
form  pearlite,  probably  does  not  have  a  tensile  strength  much 
greater  than  5,000  Ibs.  per  square  inch.  Its  ordinary  occurrence 
in  slowly  cooled  steels  (carbon  greater  than  0.9  per  cent.)  is  either 
as  a  network,  such  as  we  have  seen,  or  as  spines  and  needles. 


Pearlite  and 
Cementite 


Pearlite 


Pearlite  and 
Ferrite 


®3$. 

>  *• 


FIG.  11. — Case-carburized  Steel,   Showing  nearly  Carbonless    Steel    (Bottom) 
Gradating  into  High-carbon  Steel  (Top).     (Weyl.) 

Static  Strength. — We  may  now  sum  up  these  facts  in  their  relation 
to  the  static  strength  of  slowly  cooled  steel  as  follows:  Free  ferrite 
has  a  minimum  tensile  strength  with  maximum  ductility;  pearlite 
has  a  maximum  tensile  strength  with  low  ductility;  free  cementite 
confers  added  hardness  and  brittleness,  with  a  consequent  lowering 
of  the  tensile  strength.  In  other  words,  by  increasing  the  amount 
of  pearlite  in  the  steel,  we  increase  the  static  strength  but  with  a 


THE  STRUCTURE  OF  STEEL 


25 


corresponding  decrease  in  the  ductility.  And  as  an  increase  in  the 
amount  of  pearlite  necessarily  means  an  increase  in  the  amount  of 
carbon,  the  effect  of  increased  carbon  will  give  the  same  results. 
This  is  shown  graphically  in  the  diagram  of  Fig.  12. 

Heat  Treatment. — Heat  treatment  in  general  consists  in  chang- 
ing or  regulating  the  structure  of  the  steel  by  various  methods  of 


Per  Cent.  Elongation. 
8  8  S 
Tensile  Strength,  Lbs.  per  Sq.  Inch 

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/ 

/ 

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X 

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'Carbon    0.2               0.4               0.6               0.8               1.0                1.2               1. 
Per  Cent.  Carbon. 

FIG.  12. — Approximate  Influence  of  Carbon  upon  the  Strength  and  Ductility 

of  Steel. 

heating  and  cooling.  By  the  term  "  structure  "  is  meant  (1)  the 
metallographic  constituents,  among  which  are  those  just  described; 
(2)  the  size  of  the  grain;  (3)  the  net-work.  In  order  to  understand 
the  nature  of  these  changes  and  their  application  it  will  be  necessary 
to  have  a  clear  understanding  of  the  mechanism  by  which  these 
changes  are  brought  about. 

Critical  Points. — The  nature  of  steel,   as  explained  before,  is 
complex.     The  structure  of  any  particular  steel  may  be  modified 


26 


STEEL  AND  ITS  HEAT  TREATMENT 


or  entirely  changed  by  various  degrees  of  heating,  and  all  of  which 
take  place  in  the  steel  while  it  is  in  the  solid  condition.  These 
structural  changes  take  place  at  temperatures  known  as  the  "  critical 
points  "  or  "  critical  ranges  "  of  the  steel.  These  critical  ranges  are 
denoted  by  the  letter  "A,"  followed  by  the  letter  "c"  (abbreviation 
for  the  French  word  "  chauffage,"  signifying  "  heating  ")  or  the 


1800 

/ 

1700 

/ 

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/ 

1 

g  1600 

\ 

. 

/ 

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J* 

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Vf 

i 

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V 

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g 

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3 

A2 

\ 

s 
1 

2 

^  

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j/ 

A  1-2-3 

1300 

la 

-v 

lb 

1200 

1100 

)                 0.20               0.40               0.60              0.80               1.00               1.20               1.40 

Per  Cent.  Carbon. 

FIG.  13. — Critical  Range  or  Carbon-Iron  Diagram. 

letter  "  r  "  ("  refroidissement  "  or  "  cooling  ")•  These  signs,  Ac  or 
Ar,  are  further  modified  by  the  numerals  1,  2  or  3,  indicating  the 
particular  point  referred  to.  Thus  Acl  would  mean  the  first  critical 
range  passed  upon  heating  the  steel  beyond  a  certain  temperature, 
and  so  forth.  These  critical  points  or  ranges  are  indicated  graphic- 
ally in  Fig.  13. 


THE  STRUCTURE  OF  STEEL  27 

In  considering  this  diagram  let  us  devote  our  attention  to  a 
certain  specific  case,  such  as  a  low-carbon  steel  with  about  0.2  per 
cent,  carbon.  We  will  also  assume  that  the  steel  is  in  the  normal 
condition  resulting  from  slow  cooling,  in  that  it  consists  of  about 
25  per  cent,  pearlite  and  about  75  per  cent,  free  ferrite.  We  will 
also  first  consider  what  is  the  influence  which  these  changes  occurring 
during  the  critical  ranges  have  upon  the  constituents  of  the  steel. 

In  the  first  place,  practically  no  change  in  the  constituents  occurs 
during  heating  until  a  temperature  corresponding  to  the  lower 
critical  range,  Acl,  is  reached,  which  is  equivalent  to  about  1330°  F. 

In  passing  through  this  critical  range  there  is  a  complete  change 
in  the  nature  and  structure  of  the  pearlite,  it  being  converted  into  an 
entirely  new  constituent  with  new  characteristics.  This  is  tech- 
nically known  under  the  generic  term  of  a  "  solid  solution,"  micro- 
graphically  called  "  Austenite."  The  excess  ferrite  remains 
unchanged. 

Solid  Solutions. — To  understand  better  the  nature  of  this  new 
component  let  us  consider  the  interaction  between  salt  and  ice. 
When  these  two  substances  are  placed  in  contact  with  each  other,  we 
know  that  under  suitable  conditions  of  temperature  the  salt  and 
ice  merge  into  one  another  and  so  pass  from  the  state  of  two  separate 
substances  or  mechanical  mixture  into  that  of  one  separate  substance 
or  brine  solution.  A  similar  process  takes  place  in  the  case  of  the 
pearlite,  except  that  the  resultant  solution  is  solid  instead  of  being 
a  liquid.  The  individual  plates  of  ferrite  and  cementite  which  have 
characterized  the  pearlite  grains  now  merge  into  one  another,  form- 
ing this  new  substance  or  constituent,  known  as  a  solid  solution. 
This  new  constituent,  save  that  it  is  a  solid  and  not  a  liquid,  has  all 
the  properties  of  a  liquid  solution.  Its  original  components  are 
merged  into  a  single  entity,  giving  a  complete  indefiniteness  of  com- 
position, and  with  entirely  new  characteristics. 

Absorptive  Power  of  Austenite. — Just  as  the  brine  solution  can 
dissolve  more  salt  or  ice  with  increased  temperature,  so  this  solid 
solution  of  iron  and  iron  carbide  possesses  the  power  of  absorbing 
more  free  ferrite  or  free  cementite.  Therefore,  as  the  temperature 
is  raised  above  that  of  the  lower  critical  range  (Acl),  and  there  being 
an  excess  of  ferrite  in  this  particular  steel  (0.2  per  cent,  carbon), 
the  solid  solution  or  austenite  begins  to  absorb  this  ferrite.  This 
continues  progressively  with  increased  temperature  until  the  upper 
critical  range,  Ac3,  is  reached,  or,  for  this  particular  steel,  a  tempera- 
ture of  about  1525°  F.  At  this  temperature  the  last  of  the  remaining 


28 


STEEL  AND   ITS  HEAT  TREATMENT 


excess  of  ferrite  is  absorbed  by  the  austenite,  so  that  above  the  upper 
critical  range  of  the  steel  the  steel  is  composed  entirely  of  austenite — 
the  solid  solution. 

These  changes  are  illustrated  graphically  in  Fig.  14.  It  will  be 
seen  that  the  initial  pearlite,  comprising  about  25  per  cent,  of  the 
normal  steel,  changes  into  austenite  (the  solid  solution)  at  a  tempera- 
ture corresponding  to  that  of  the  lower  critical  range,  Acl,  and  then 
progressively  absorbs  the  free  ferrite  until  at  a  temperature  corre- 
sponding to  that  of  the  upper  critical  range,  Ac3,  the  whole  steel  con- 
sist^ of  austenite. 


AcS 


Y 


Free  Fcrrite 


FIG.  14. — Change  of  Pearlite  and  Free  Ferrite  into  Austenite  during  Heating. 
Carbon  about  0.2  per  cent. 

These  same  changes  are  shown  microscopically  in  Figs.  15,  16, 
17  and  18.  The  first  photomicrograph  shows  the  normal  condition 
of  the  steel,  being  made  up  of  a  small  proportion  of  pearlite  (dark), 
and  a  large  amount  of  free  ferrite  (light) .  The  three  other  structures 
were  obtained  by  heating  this  same  steel  to  temperatures  above  the 
lower  critical  range  and  then  "  fixing  "  the  structure  obtained  at 
those  temperatures  by  "  quenching."  Fig.  16  shows  the  structure 
representative  of  a  temperature  between  that  of  the  Acl  and  Ac2, 
the  solid  solution l  (dark)  having  increased  considerably  in  amount 

1  Strictly  speaking,  the  dark  areas  thus  referred  to  as  the  "  solid  solution  " 
are  not  austenite,  but  its  transitional  stage,  martensite.  In  the  ordinary  carbon 
steels  austenite  as  such  cannot  be  retained  by  the  ordinary  methods  of  quenching 


THE   STRUCTURE  OF  STEEL 


29 


over  that  of  the  original  pearlite  as  in  the  previous  figure.  Fig.  17 
represents  the  structure  obtained  at  a  temperature  somewhat  under 
that  of  the  upper  critical  range,  Ac3;  in  this  case  it  will  be  noted 
that  the  solid  solution  covers  nearly  the  whole  field,  there  being  but  a 
small  amount  of  the  free  ferrite  (white).  The  structure  representa- 
tive of  heating  to  slightly  above  the  upper  critical  range  is  shown  in 
Fig.  18;  it  will  be  seen  that  the  free  ferrite  has  now  been  entirely 
absorbed  by  the  solid  solution.  Also  note  the  extremely  refined 


FIG.  15. — Normal  Low-carbon  Steel  as  Rolled.     X60.     (Bullens.) 


structure,  as  we  shall  have  occasion  to  refer  to  this  particular  feature 
a  little  later. 

Allotropic  Modifications  of  Ferrite. — Associated  with  these 
critical  ranges  there  is  also  a  change  in  the  allotropic  1  form  of  the 
ferrite  (iron).  Thus  pure  ferrite  (as  distinguished  from  the  ferrite 

(as  will  be  explained  under  the  chapter  on  Hardening),  but  changes  into  martens- 
ite.  Martensite,  however,  is  also  a  solid  solution,  and  for  the  purposes  of  explana- 
tion in  this  chapter — in  order  not  to  complicate  matters — we  will  consider  it 
permissible  to  use  the  term  as  indicated. 

1  Sauveur  defines  allotropy  as  "  suggesting  marked  and  sudden  changes  in 
some  of  the  properties  of  a  substance  occurring  at  certain  critical  temperatures, 
without  any  change  of  state  or  of  chemical  composition." 


30 


STEEL  AND   ITS  HEAT  TREATMENT 


FIG.  16. — Low-carbon  Steel  Quenched  between  Acl  and  Ac2.     X60.     (Bullens.). 


FIG.  17.— Low-carbon  Steel  Quenched  a  Little  below  Ac3.     X60,     (Bullens.) 


THE  STRUCTURE  OF  STEEL  31 

associated  with  cementite  to  form  pearlite)  in  its  normal  condition 
is  called  "  alpha  "-ferrite  or  "  alpha  "-iron,  and  is  characterized 
by  extreme  ductility  and  magnetic  properties.  Upon  heating  this 
alpha-ferrite  to  a  little  over  1400°  F.,  corresponding  to  the  critical 
range  Ac2,  the  iron  becomes  practically  non-magnetic  and  is  then 
known  as  "  beta  "-ferrite  or  "  beta  "-iron.  Upon  further  heating  to 
a  temperature  above  the  upper  critical  range,  Ac3,  there  is  still 
another  change  in  the  allotropic  modification  of  the  iron,  it  being 
known  as  "  gamma  "-ferrite;  this  gamma-iron  is  slightly  softer  than 
the  beta  modification.  Gamma-iron  has  the  property  of  being  able 


FIG.  18.— Low-carbon  Steel  Quenched  above  A3.     X60.     (Bullens.) 

to  dissolve  carbon  or  iron  carbide,  a  characteristic  which  is  not  held 
by  alpha-iron. 

Merging  of  the  Critical  Points. — Now  by  referring  to  the  carbon- 
iron  diagram  in  Fig.  13  it  will  be  noted  that  at  the  eutectoid  ratio  of 
carbon,  that  is,  at  about  0.9  per  cent,  carbon,1  the  three  critical  ranges 
Al,  A2,  and  A3,  merge  into  one.  That  is,  steels  consisting  of  pearlite 
alone,  when  heated  to  a  temperature  beyond  this  point,  will  change 
directly  into  the  solid  solution  austenite,  which  will  consist  of  a 
solution  of  carbide  (or  carbon,  according  to  some  authorities)  in 

1  The  eutectoid  ratio  on  the  chart  is  given  as  0.85  per  cent,  carbon.  Accord- 
ing to  the  authority  selected  this  ratio  will  vary  between  0.8  and  0.9  per  cent, 
carbon;  but  the  more  recent  tendency  is  to  adopt  0.90  per  cent. 


32  STEEL  AND   ITS  HEAT  TREATMENT 

gamma-iron.  Similarly,  as  normal  pearlite  always  represents  this 
eutectoid  ratio,  the  same  change  of  pearlite  into  a  solid  solution  of 
carbide  in  gamma-iron  will  always  occur  at  this  temperature  in  ordi- 
nary carbon  steels  irrespective  of  the  carbon  content  of  the  steel  as 
a  whole. 

Changes  in  Heating  Different  Steels. — With  this  explanation 
clearly  in  mind,  we  may  now  refer  back  to  the  example  of  the  0.2 
per  cent,  carbon  steel  and  more  fully  explain  the  changes  which  take 
place  in  the  constituents.  Under  normal  conditions,  this  steel  will 
consist  of  pearlite  plus  alpha-ferrite.  Upon  heating  through  the 
Acl  range,  the  pearlite  will  change  into  austenite,  the  iron  of  which 
will  be  in  the  gamma  modification;  the  free  ferrite  will  still  remain 
in  the  alpha  condition.  Upon  further  heating  through  the  zone 
marked  "  2  "  on  the  diagram  Fig.  13,  the  austenite  will  begin  to 
absorb  the  free  ferrite.  Upon  passing  through  the  Ac 2  range  the 
balance  of  the  free  ferrite  will  pass  from  the  alpha  modification  into 
that  of  beta-f errite ;  the  steel  as  a  whole  will  be  hard  and  non-mag- 
netic. Upon  further  heating  (zone  3)  the  remnant  of  the  beta  free 
ferrite  will  be  gradually  absorbed,  so  that  on  passing  through  the 
critical  range,  Ac3,  the  whole  steel  will  be  in  the  condition  of  austen- 
ite (zone  5),  or  a  solution  of  iron  carbide  (or  carbon)  in  gamma-iron. 

In  a  similar  manner  we  might  explain  the  changes  in  constituents 
which  take  place  upon  heating  normal  steel  with  any  carbon  up 
to  that  of  the  eutectoid  ratio.  With  a  carbon  content  somewhere 
between  0.3  and  0.4  per  cent,  (varying  according  to  different  author- 
ities) it  will  be  noted  that  the  A2  and  A3  ranges  merge  into  one, 
known  as  A2-3. 

In  a  manner  analogous  to  the  absorption  of  free  ferrite  by  the 
solid  solution  in  the  hypo-eutectoid  steels,  the  free  cementite  will  be 
absorbed  in  the  case  of  the  hyper-eutectoid  steels,  the  final  solution 
taking  place  at  a  temperature  range  indicated  by  the  line  Acm. 
The  only  difference,  and  that  a  practical  one,  is  that  the  solution  of 
the  free  cementite  takes  place  more  sluggishly  than  the  solution  of 
the  free  ferrite  of  the  lower  carbon  steels. 

The  Ar  Ranges. — Corresponding  critical  changes  take  place 
upon  cooling  slowly  from  above  the  upper  critical  range,  except  that 
they  occur  in  the  reverse  order  and  with  opposite  effect.  On  account 
of  the  molecular  inertia,  however,  we  find  that  these  critical  ranges 
(of  cooling,  Ar3,  Ar2,  Arl,  etc.)  are  a  number  of  degrees  below  the 
temperatures  at  which  they  appeared  on  heating.  This  difference 
is  dependent  upon  length  of  exposure  and  the  temperature  to  which 


THE  STRUCTURE  OF  STEEL  33 

the  steel  was  subjected,  the  rate  of  cooling,  and,  more  particularly, 
upon  the  influence  of  the  alloying  elements  which  may  have  been 
added  to  the  steel.  Some  of  the  alloys,  if  present  in  sufficient  amount, 
will  cause  the  recalescent  points  to  fall  below  normal  temperatures, 
and  are  the  basis  of  air-hardening  steels  and  similar  compositions. 

Changes  on  Slow  Cooling. — Upon  slow  cooling  from  above  the 
upper  critical  range,  the  solid  solution  will  commence  to  reject  the 
excess  ferrite  (or,  of  course,  the  excess  cementite  in  the  case  of  hyper- 
eutectoid  steels)  as  the  temperature  decreases  from  Ar3  to  Arl. 
The  reverse  changes  in  the  physical  nature  and  properties  of  the 
iron  occur  at  the  critical  ranges  during  cooling  as  those  previously 
noted  under  heating.  When  the  lower  critical  range  is  reached,  the 
excess  ferrite  or  cementite  will  have  been  entirely  rejected,  and  as 
the  steel  passes  downward  through  this  range  (or  point),  the  solid 
solution — now  containing  0.9  per  cent,  carbon — will  change  into 
pearlite.  Under  similar  conditions  of  cooling,  the  original  steel  and 
the  present  heated  and  cooled  steel  will  have  the  same  structure. 

Refinement. — Before  leaving  the  subject  of  the  influence  which 
heating  through  these  various  critical  ranges  has  upon  the  structure 
of  the  steel,  there  are  a  few  points  which  we  wish  to  mention  briefly 
concerning  refinement.  Again  assuming  that  the  steel  is  in  the 
normal  condition,  no  change  will  take  place  in  the  structure  until  the 
temperature  has  been  raised  at  least  to  that  of  the  lower  critical 
range.  At  this  temperature  the  original  pearlite  grains  are  com- 
pletely changed  and  will  possess  that  maximum  refinement  which 
the  formation  of  the  austenite  can  impart — that  is,  complete  refine- 
ment. If  the  steel  has  a  carbon  content  other  than  that  of  the  eutec- 
toid  ratio  (i.e.,  contains  free  ferrite  or  free  cementite),  the  steel  as  a 
whole  will  not  be  refined;  the  excess  ferrite  or  cementite  will  remain 
unaltered  and  the  steel  will  retain  its  original  grain-size.  This  is 
brought  out  by  a  comparison  of  Figs.  15  and  16.  Complete  refine- 
ment of  the  steel  as  a  whole  will  not  result  until  the  steel  has  been 
heated  to  a  temperature  slightly  over  that  of  the  upper  critical  range, 
as  a  comparison  of  Figs.  17  and  18  will  prove,  and  as  is  evident  from 
previous  discussion.  A  clear  understanding  of  these  principles  must 
be  had,  as  they  form  the  basis  of  many  of  the  heat  treatment  pro- 
cesses which  will  be  later  developed. 

Grain-Size  Beyond  Ac3. — As  the  temperature  is  progressively 
raised  above  the  critical  range,  a  gradual  coarsening  of  the  aus- 
tenite grains  occurs.  This  increase  size  is  not  only  a  function  of  the 
temperature,  but  also  of  the  length  of  time  at  which  the  high  tern- 


34  STEEL  AND  ITS  HEAT  TREATMENT 

perature  is  maintained.  The  practical  application  of  the  principles 
noted  in  this  and  the  previous  sections  will  be  considered  in  the  chap- 
ter on  Annealing. 

Network. — The  third  factor  in  the  structural  changes  taking 
place  upon  heating  is  the  effect  of  temperature  upon  the  network. 
All  hypo-eutectoid  steels  in  the  normal  condition  are  made  up  of 
pearlite  with  a  varying  amount  of  excess  ferrite,  the  latter  decreasing 
with  the  increase  in  carbon  content.  From  our  study  of  the  inter- 


FIG.  19. — Microstructure  of  Cast-steel  Ingot  as  Cast.  X75.  (Ordnance  Dept.) 
Tensile  Strength,  77,000.  Elastic  Limit,  39,000.  Elongation,  10.5. 
Red.  of  Area,  16.9. 


nal  mechanism  by  which  the  constituents  of  the  steel  are  formed 
by  slow  cooling,  we  know  that  the  pearlite  forms  the  basis  of  the 
structure,  the  ferrite  being  rejected  by  the  solid  solution  (pre-pearlite). 
Being  thrown  out  to  the  boundaries  of  these  austenitic  grains,  the 
excess  ferrite  forms  a  network  around  these  grains.  Upon  reheat- 
ing, this  network  is  gradually  absorbed,  its  final  absorption  taking 
place  upon  passing  the  upper  critical  range.  This  change  is  similar 
to  that  explained  previously  under  the  description  of  the  action  of 
the  excess  ferrite. 


THE  STRUCTURE  OF  STEEL 


35 


FIG.  20.— Microstructure  of  Cast  Steel  Ingot  Forged  to  1450°  F.  X75.  (Ord- 
nance Dept.)  Tensile  Strength,  83,500.  Elastic  Limit,  50,500. 
Elongation,  27.5.  Red.  of  Area,  43.3. 


FIG.  21. — Microstructure  of  Steel  Subjected  to  Cold  Work,  and  Showing  Dis- 
tortion of  Grain.     X50.     (Ordnance  Dept.) 


36 


STEEL  AND   ITS   HEAT   TREATMENT 


FIG.  22. — Hammer-hardened  Steel,  0.46  per  cent.  Carbon.     X300.     (Savoia.) 


FIG.  23.— Effect  of  Cold  Rolling  on  0.20  per  cent.  Carbon  Steel.     X60. 

(Bullens.) 


THE   STRUCTURE   OF   STEEL 


37 


FIG.  24. — Effect  of  Punching  upon  Structure    of    &-in.  Chrome-nickel    Steel 
Plate.     Hole  Downwards  and  at  Right.     X50.     (Bullens.) 


FIG.  25. — Machining  Strains  on  Surface  of  Mild  Steel.     (Brearley.) 


38  STEEL  AND  ITS  HEAT  TREATMENT 

The  Effect  of  Work  on  Grain-Size.1 — Steel  cooled  slowly  and 
undisturbed  from  a  high  temperature  will  show  a  coarsely  granular 
or  crystalline  structure,  and  the  size  of  the  grain  is  a  function  of  the 
temperature  and  time  during  which  the  material  is  held  at  the  maxi- 
mum temperature,  and  the  rate  at  which  the  material  is  cooled.  In 
large  masses  of  material  the  structure  will  be  coarser  in  the  center 
than  at  the  surface,  due  to  the  difference  in  rate  of  cooling.  In 
order  to  overcome  this  difference  and  at  the  same  time  produce  a 
homogeneous,  uniform  material,  the  steel  is  worked  during  the  period 
at  which  grain  growth  would  ordinarily  take  place.  Steel  which  has 
been  hot- worked  down  to  the  Arl  point  will  show  a  finer  grain,  and 
will  be  stronger  than  the  same  steel  slowly  cooled  without  work, 
and  will  at  the  same  time  show  high  ductility.  Examples  of  steel 
worked  and  unworked  are  shown  in  the  photomicrographs  of  Figs. 
19  and  20. 

Steel  which  has  been  worked  below  the  Arl  range — that  is,  cold- 
worked — will  show  considerable  distortion  of  grain,  as  is  illustrated 
by  Fig.  21,  and  may  even  become  hardened,  Fig.  22.  Cold  rolling 
frequently  develops  a  weak,  laminated  structure,  as  is  shown  in  Fig. 
23.  Even  punching  or  machining  operations  may  greatly  affect 
the  structure,  examples  of  which  are  given  in  Figs.  24  and  25. 

1  In  part  from  Bulletin  1961,  Ordnance  Dept. 


CHAPTER  III 
ANNEALING 

Annealing. — Annealing,  in  its  commercial  application,  may  have 
for  its  purpose  any  or  all  of  the  following  aims:  (1)  to  "  soften  "  the 
steel  and  thus  put  it  in  condition  for  machining  or  to  meet  certain 
physical  specifications;  (2)  to  relieve  any  internal  stresses  or  strains 
caused  by  previous  hardening  or  elaborating  operations;  (3)  to  obtain 
the  maximum  refinement  of  the  grain  in  combination  with  large 
ductility. 

Thus,  depending  upon  the  results  desired,  commercial  annealing 
will  consist  of  a  heating  operation  carried  to  some  predetermined 
temperature — although  not  necessarily  over  the  critical  range — to 
produce  the  results  desired  in  items  1  and  2  previously  noted,  and 
followed  by  a  moderately  slow  cooling  of  the  metal  from  that  tem- 
perature. True  or  full  annealing  requires  a  heating  to  above  the 
upper  critical  range  of  the  steel. 

Elemental  Considerations. — In  the  abstract,  annealing  would 
appear  to  be  but  a  suitable  correlation  of  the  following  elements: 

1.  Rate  of  heating; 

2.  Temperature  of  heating; 

3.  Length  of  heating; 

4.  Rate  of  cooling. 

But  in  actual  practice  the  success  to  be  attained  in  annealing  (or  in 
any  heat  treatment  process,  for  that  matter)  must  depend  upon 
the  judgment  and  skill  of  the  furnace  operator  in  applying  the  basic 
principle's  which  may  be  derived  from  a  consideration  of  the  above 
factors.  Thus  it  is  the  man  who  determines  the  manner  of  placing 
the  charge  in  the  furnace,  of  regulating  the  flow  and  composition  of 
the  hot  gases,  of  determining  when  the  steel  has  been  uniformly 
and  thoroughly  heated,  and  similar  fundamentals.  For  while  it  is 
advisable  and  perhaps  necessary  to  understand  the  theory  behind 
the  actual  work  itself — the  "  wherefore  " — the  "  wherewithal  "  is 
largely  a  personal  equation  and  should  be  borne  in  mind  throughout 
every  theoretic  discussion  of  principles  or  practice. 

39 


40  STEEL  AND   ITS  HEAT  TREATMENT 

Heat  Application. — The  manner  in  which  the  steel  is  placed  in 
the  furnace  is  a  factor  of  supreme  importance.  It  may  even  be 
said  that  three  different  kinds  of  annealing  may  be  produced  in  the 
same  furnace  operating  at  the  same  indicated  pyrometer  reading, 
dependent  simply  upon  the  method  of  placing  the  sto\;k  in  the  furnace. 
Particular  stress  should  be  laid  upon  the  necessity  for  getting  heat 
to  the  center  and  bottom  of  the  charge,  not  only  for  the  sake  of 
uniform  annealing,  but  also  to  shorten  the  time  of  absorption  and 
lessen  the  time  of  exposing  the  top  and  outside  edges  of  the  charge 
to  the  heat  and  influences  of  the  chamber  atmosphere.  Thus  it  has 
been  shown  that  a  uniform  chamber  temperature  does  not  necessa- 
rily mean  a  uniformly  annealed  product;  that  a  circulation  of  heat 
through  the  mass  is  more  desirable  than  the  mere  application  from 
the  outside;  that,  with  the  same  chamber  uniformity,  it  is  possible 
to  vary  the  quality  of  the  anneal  by  the  manner  in  which  the  stock  is 
placed  in  the  annealing  zone.  It  is  advisable  to  raise  the  charge 
above  the  furnace  floor  or  hearth  upon  suitable  blocks  or  supports, 
to  separate  each  piece  from  the  other,  and  to  avoid  localized  heating 
through  over-loading.  It  is  only  by  such  means  that  there  will  be 
provided  an  opportunity  for  the  circulation  of  the  hot  gases  through 
the  charge. 

Pre-Heating. — Slow,  careful  and  uniform  heating  is  always 
advisable  regardless  of  the  chemical  composition  or  physical  condi- 
tion of  the  steel.  Heating  to  such  temperatures  as  are  common  in 
general  annealing  practice  necessarily  results  in  more  or  less  change 
of  physical  condition  or  molecular  readjustment,  and  the  greater 
the  hardness,  brittleness  and  amount  of  internal  strain  in  the  metal, 
the  greater  will  be  the  deleterious  effect  of  such  heating.  Thus 
objects  of  intricate  design,  or  with  varying  cross-sections,  or  steel 
in  a  hard,  brittle  condition,  should  be  given  the  greatest  care  in  heat- 
ing in  order  that  the  release  of  any  strains  shall  not  cause  warpage 
or  otherwise  injure  the  metal.  Such  pieces  should  never  be  placed 
directly  in  a  hot  furnace,  but  should  be  given  a  careful  pre-heating. 

ANNEALING    HYPO-EUTECTOID    STEELS 

Microscopic  Changes. — In  the  previous  chapter  we  have  explained 
that,  in  the  ordinary  cast,  rolled  or  forged  sections  (pearlitic  in 
character),  there  is  virtually  no  change  in  grain  size  or  in  constit- 
uents during  the  heating  to  a  temperature  below  that  of  the  lower 
critical  range  Acl.  That  is,  there  is  no  refinement  of  the  steel. 


ANNEALING  41 

As  the  temperature  passes  the  Acl  range  there  occurs  the  complete 
change  of  the  pearlite  to  the  solid  solution,  giving  the  maximum 
refinement  to  the  austenite. 

Passing  through  zone  2  (Refer  to  Fig.  13)  the  excess  ferrite  is 
progressively  absorbed  by  the  solid  solution,  causing  an  apparent 
decrease  in  the  grain-size  of  the  steel  as  a  whole.  This  absorption 
is  the  slower  the  greater  the  carbon  until  the  carbon  nears  0.85 
per  cent.,  but  is  offset  by  the  fact  that  the  amount  of  free  ferrite 
decreases  as  the  eutectoid  ratio  is  approached. 

Upon  passing  through  the  critical  range  Ac2  we  have  the  forma- 
tion of  beta  iron  with  no  apparent  change  between  the  relative 
grain-size  of  the  alpha  ferrite  and  beta  ferrite  grains.  The  same 
absorption  of  the  excess  ferrite  continues  progressively,  but  with 
increased  sluggishness  (due  to  the  supposed  properties  of  beta  ferrite) . 
This  applies  to  steels  with  say  0.12  to  0.30  per  cent,  carbon.  In 
the  very  low  carbon  steels  Howe  1  sums  up  the  probable  changes 
during  this  period  in  a  provisional  proposition  that  t»>)  if  initially 
fine-grained  the  steel  coarsens,  though  only  very  slowly;  (b)  if 
initially  coarse-grained  it  refines  slowly;  (c)  to  coarsen  again  upon 
long  exposure  to  these  temperatures. 

The  changes  taking  place  through  zone  2  continue  through 
zone  3,  although  more  slowly.  If  the  rate  of  heating  through  this 
range  of  temperatures  is  comparatively  slow,  there  will  be  a  complete 
absorption  of  the  remaining  ferrite  just  before  Ac3  is  reached. 
Under  ordinary  circumstances  final  absorption  will  occur  on  passing 
through  the  Ac3  range. 

The  Upper  Critical  Range. — As  the  steel  passes  the  upper  critical 
range  there  is  the  complete  refining  of  the  grain,  it  becoming  very 
fine  and  almost  amorphous.  As  the  temperature  increases  beyond 
this  range  the  grain-size  coarsens,  causing  a  diminution  in  the 
strength  of  the  steel.  The  effect  upon  the  physical  properties  of 
the  steel  is  great.  The  tensile  strength  is  increased  somewhat  as 
the  temperature  advances.  The  elastic  limit  rises  until  a  point  is 
reached  about  175°  to  200°  F.  over  the  upper  critical  range,  after 
which  it  then  decreases.  The  elongation  and  reduction  of  area 
decrease  very  rapidly.  These  changes  in  the  physical  properties 
are  shown  graphically  in  Fig.  26  in  which  the  results  obtained  by 
heating  a  0.40  per  cent,  carbon,  basic  open-hearth  steel  to  a  definite 
temperature  and  then  slow-cooling  with  the  furnace  are  plotted 

1  H.  M.  Howe,  "Life  History  of  Network  and  Ferrite  Grains  in  Carbon  Steel," 
Proc,  A,  S,  T,  M,,  Vol.  XI,  1911. 


42 


STEEL  AND  ITS  HEAT  TREATMENT 


against  the  temperatures.  It  will  be  noted  that  the  softest  and 
most  ductile  steel  is  obtained  at  approximately  1475°  F.,  which  is 
about  50°  over  the  upper  critical  range. 

Heating  Over  the  Upper  Critical  Range. — The  effect  of  heating 
beyond  the  critical  range  is  well  developed  by  the  series  of  photo- 
graphs (by  Howe)  shown  in  Figs.  27,  28,  29,  30  and  31.  The  steel 
(0.40  per  cent,  carbon,  0.16  per  cent,  manganese)  was  heated  to  the 


1,200  1,300  1,400  1,500  1,600  1,700  1,800 

Degrees  Fahrenheit 

FIG.  26. — Effect  of  Annealing  Temperature  on  Physical  Properties. 

temperatures  indicated,  held  at  those  temperatures  for  ten  minutes, 
and  then  cooled  in  air.  There  is  a  difference  in  grain-size  between 
that  cooled  from  1472°  F.  and  from  1652°  F.,  showing  that  anneal- 
ing should  never  be  carried  very  far  beyond  the  upper  critical  range 
or  Ac3  point  unless  for  special  reasons.  As  the  high  temperatures 
were  successively  raised  to  1832°  and  2012°  the  grain-size  becomes 
noticeably  larger,  until  at  2192°  the  steel  is  "  burnt."  These 
photomicrographs  also  exhibit  the  effect  of  air  cooling  upon  the 
structure,  in  that  it  develops  a  distinct  net-work  or  cellular  structure. 
The  effect  of  heating  beyond  the  upper  critical  range  is  also  brought 


ANNEALING 


43 


EFFECT  OF  HEATING  BEYOND  Ac3, 

0.40  per  cent.  Carbon  Steel  Heated  at  Temperatures  Indicated  for  Ten  Minutes  and 

AIR  COOLED.  • 


FIG.  27.— 1472°  F.     X40.     (Howe.)        FIG.  28.— 1652°  F.     X40.     (Howe.) 


FIG,  29,— 1832°  F.     X40.    (Howe.)        FIG.  30.— 2012°  F.     X40,     (Howe.) 


FIG.  31.— 2192°  F.     X40.     (Howe.) 


44  STEEL  AND   ITS  HEAT  TREATMENT 

out  in  an  analogous  manner  by  Figs.  32,  33,  34,  35  and  36,  except 
that  in  this  case  the  steel  has  been  cooled  very  slowly  (furnace  cooled) 
from  the  specific  temperatures. 

Use  of  the  Microscope  for  Checking  Structural  Changes. — From 
previous  theoretical  discussion,  it  is  evident  that  in  order  to  fulfill  the 
true  or  full  annealing  operation,  it  is  necessary  to  heat  the  metal  to 
over  the  upper  critical  range  of  the  steel  in  order  to  obtain  the  com- 
plete change  of  structure  with  the  smallest  grain-size  possible.  The 
microscopic  changes  which  take  place  during  such  heating  of  a  0.28 
per  cent,  carbon  steel  are  given  as  follows: 

Temperature.  Structure. 

1325°  F.        Very  coarse  ferrite  and  pearlite  similar  to 

the  original  bar. 
1375°  F.        Laminae    of    ferrite     strong,    ground-mass 

refined. 

1425°  F.        About  25  per  cent,  ferrite  laminae  left. 
1475°  F.         Trace  of  coarse  ferrite  unabsorbed. 
1500°  F.         Complete  refining.     No  coarse  ferrite. 
1550°  F.         Structure  similar. 
1650°  F.         Refined  but  grain-size  coarsening. 

These  experiments  l  were  carried  out  with  a  view  to  discover  the 
cause  of  failure  of  an  eye-bar  (carbon  0.28  per  cent.)  when  placed 
in  service.  The  original  steel  had  been  annealed  several  times  at 
temperatures  under  the  upper  critical  range,  but  a  microscopic  study 
showed  that  these  heatings  had  simply  refined  the  pearlitic  ground- 
mass.  In  other  words,  it  was  found  that  the  proper  annealing 
temperature  necessary  to  obtain  a  completely  refined  steel  was 
beyond  the  upper  critical  range.  In  this  steel  it  would  seem  to  be 
about  1500°  F.  The  lower  critical  range  is  shown  by  the  refining 
of  the  ground-mass  which  occurred  between  1325°  and  1375°. 

Diffusion. — We  have  repeatedly  stated  that  complete  absorption 
of  the  excess  ferrite  takes  place  at  the  upper  critical  range  of  the  steel. 
Although  this  statement  is  true,  there  is  another  phase  of  this 
absorption  to  be  considered,  and  a  full  understanding  of  which  will 
probably  clear  up  many  of  the  questions  which  have  perplexed 
those  unfamiliar  with  the  theory  of  annealing.  This  phenomenon 
may  be  called  "  diffusion."  Let  us  hark  back  to  our  former  simile 

i  Wm.  Campbell,  "  Further  Notes  on  the  Annealing  of  Steel,"  Proc.  A.  S.  T. 
M.,  Vol.  X,  1910. 


ANNEALING 


45 


EFFECT  OF  HEATING  BEYOND  Ac3. 

0.40per  cent.'Carbon  Steel  Heated  at  Temperatures  Indicated  for  Ten  Minutes  and 

FURNACE  COOLED. 


FIG.  32.— 1472°  F.     X40.    (Howe.)        FIG.  33.— 1652°  F.     X 40.  .  (Howe.) 


FIG.  34.— 1832°  F.     X40.    (Howe.)        FIG.  35.— 2012°  F.     X40,     (Howe.) 


FIG.  36.— 2192°  F.     X40.    (Howe.) 


^46  STEEL  AND  ITS  HEAT  TREATMENT 

of  the  salt  and  brine  solution.  When  a  grain  of  salt  is  dissolved  by 
the  brine,  it  is  the  solution  in  the  immediate  neighborhood  of  the 
salt  crystal  which  acts  as  the  solvent  and  not  the  entire  volume  of 
the  brine  solution.  In  time,  however,  the  dissolved  salt  will  eventu- 
ally diffuse  through  the  whole  body  of  brine  and  the  brine  will  then 
be  of  equal  composition  throughout.  Now  a  similar  process  is  going 
on  in  the  steel  when  the  solid  solution  (austenite)  is  absorbing  the 
excess  ferrite,  and  it  will  be  found  that  complete  absorption  may  not 
mean  complete  diffusion  or  equalization.  The  process  of  equalization 
goes  on  with  the  rise  in  temperature.  If  the  passage  through  tem- 
peratures under  that  of  the  upper  critical  range  is  only  slow  enough, 
a  large  part  of  the  diffusion  will  have  occurred  by  the  time  Ac3  is 
reached.  In  order  that  there  may  be  complete  diffusion,  and  there- 
fore complete  grain-refining,  the  sojourn  at  a  temperature  approxi- 
mating Ac3  must  be  long  enough  for  this  complete  diffusion  of  the 
absorbed  excess  ferrite  and  therefore  of  the  solid  solution.  Although 
exposure  to  a  higher  temperature  would  naturally  hasten  this  diffu- 
sion, it  would  be  at  a  cost  of  coarsening  the  austenite  grains.  The 
effects  of  non-equalization  will  be  discussed  in  a  later  part  of  the 
chapter. 

Rate  of  Heating. — Studying  the  rate  of  heating  from  the  practical 
aspect  there  is  also  another  factor  to  be  considered — that  of  bringing 
the  whole  mass  of  the  steel  to  the  proper  temperature  evenly.  It 
is  self-evident  that  the  center  of  a  large  mass  of  steel,  such  as  loco- 
motive axles  or  steel  blooms,  will  lag  in  temperature  behind  the 
exterior.  In  other  words,  it  is  the  tendency  of  the  core  to  be  con- 
siderably lower  in  temperature  than  the  shell  or  outside  of  the  steel. 
It  is  then  a  common  procedure  to  raise  the  temperature  of  the  fur- 
nace beyond  the  proper  annealing  heat  in  order  to  drive  the  heat 
to  the  center  of  the  piece  to  be  annealed.  This  is  a  great  mistake. 
It  is  far  better  to  take  the  extra  time  required  to  heat  more  slowly 
as  the  proper  temperature  is  neared,  thus  bringing  the  steel  to  an 
even  temperature  throughout.  If  this  were  not  done,  the  exterior 
of  the  piece  might  be  carried  beyond  the  proper  temperature — and,  in 
general,  a  needlessly  high  temperature  is  injurious  and  tends  to 
recoarsen  the  grain. 

Expressing  this  question  in  a  different  way,  we  may  say  that  the 
furnace  in  which  the  metal  is  being  heated  for  annealing  should  in 
no  case  be  run  at  a  higher  indicated  temperature  than  the  maximum 
temperature  to  which  the  metal  itself  is  to  be  heated.  To  illustrate: 
a  piece  of  steel  heats,  cools  and  decarbonizes  on  the  corners  first. 


ANNEALING 


47 


The  life  of  the  entire  piece  of  steel  is  no  greater  than  the  life  of  the 
corners.  If  the  steel  is  placed  in  a  hot  furnace,  the  corners  are  apt 
to  be  heated  long  before  the  major  part  of  the  mass.  If  the  temper- 
ature is  high,  the  corners  are  overheated  before  the  center  of  the 
mass  is  saturated.  From  this  commonplace  example  there  should 
be  indicated  the  necessity  for  slow,  soaking  heats  in  order  to  prevent 
overheating  the  corners  of  the  metal,  and  further,  the  necessity  of 
soft  hazy  heats  to  prevent  oxidation  or  decarburization  of  the 
exposed  edges. 

Temperature  of  Heating. — Assuming  that  the  proper  degree  of 
care  has  been  used  in  heating  the  steel,  the  next  question  is  the  degree 
of  heat  necessary.  Reduced  to  lowest  terms,  the  true  or  full  anneal- 
ing operation  requires  the  production  of  an  entirely  new  crystalline 
structure,  the  constituents  of  which  shall  be  of  the  smallest  grain- 
size  attainable;  this  operation  should  also  eliminate  all  internal 
strains  and  stresses.  As  previously  described,  this  new  structure 
is  given  birth  at  a  temperature  known  as  the  "  upper  critical  range  " 
of  the  steel.  The  exact  temperature  1  will  depend  upon  the  chemical 
composition  of  the  steel,  and,  more  particularly,  upon  the  carbon 
content.  As  this  transformation  does  not  occur  suddenly,  but 
usually  covers  a  range  of  some  25°  to  50°  it  is  customary  to  adopt 
a  temperature  of  about  50°  over  the  upper  critical  range  as  the 
proper  annealing  heat.  For  straight-carbon  steels  these  may  be 
roughly  given  as  shown  in  the  chart  in  Fig.  37.  The  upper  critical 
range  is  approximately  located  by  the  dash  line  on  the  chart. 

The  temperatures  recommended  by  the  American  Society  for 
Testing  Materials  2  are  as  follows : 


Range  of  Carbon  Content. 

Range  of  Annealing 
Temperature. 

Less  than  0.12  per  cent. 
0.12  to  0.29 
0.30  to  0.49 
0.50  to  1.00 

1607°  to  1697°  F. 
1544°  to  1598°  F. 
1499°  to  1544°  F. 
1454°  to  1499°  F. 

1  Methods  for  determining  the  critical   ranges  are  described  in  Chapter 
XVIII. 

2  It  will  be  noticed  that  the  temperatures  recommended  by  A.  S.  T.  M.  are 
distinctly  higher — especially  for  the  tool-steel  grades — than  those  advised  by 
the  author.     In  the  light  of  my  own  experience,  and  that  of  others,  I  believe 
that  the  lower  the  temperature  which  can  be  used  to  give  the  desired  results, 
the  greater  will  be  the  maximum  efficiency  of  the  annealed  steel. 


48 


STEEL  AND  ITS  HEAT  TREATMENT 


Length  of  Heating. — Ordinarily  the  underlying  practice  of  this 
part  of  the  operation  is  to  heat  the  steel  until  the  whole  mass  has 
been  heated  uniformly  throughout  at  the  proper  temperature.  This 
will  of  course  depend  upon  the  size  of  the  object.  This  full  heating  is 
generally  sufficient  to  give  birth  to  the  new  grain  structure  and  relieve 
all  internal  stresses.  The  proper  rate  of  cooling  should  then  maintain 
the  steel  in  that  condition.  If  the  steel  should  be  quenched  in  some 
hardening  bath  such  as  oil  or  water,  this  new  grain-size  and  rear- 


iroo 


1200 


o.i 


0.3  0.4  0.5  0.6 

Carbon  Content,  Per  Cent. 


0.9 


FIG.  37. — Annealing  Range  for  Carbon  Steels, 


rangement  of  the  structure  would  be  kept.  The  annealing  operation 
should  theoretically  bring  about  approximately  the  same  results  as 
to  grain-size,  neglecting  for  the  moment  the  effect  of  slow  cooling 
through  the  transformation  ranges.  From  the  standpoint  of  prac- 
tice, however,  much  difficulty  is  experienced  in  this  regard,  par- 
ticularly in  cases  where  the  mechanical  work  upon  the  steel  has  been 
severe,  and  also  in  alloy  steels. 

It  seems  that  the  greater  the  internal  stress  upon  the  steel  the 
greater  is  the  amount  of  intermolecular  lag  or  final  release  of  this 


ANNEALING  49 

stress  behind  the  actual  change  of  constituents.  That  is,  even  though 
a  totally  new  structure  may  be  set  up  by  the  annealing  temperature, 
there  remains  for  a  considerable  length  of  time  a  tendency  of  the  new 


FIG.  38.— Frame  Steel  as  Rolled.     X60.    (Bullens.) 

structure  to  return,  upon  slow  cooling,  to  the  stressed  condition  of 
the  original,  even  though  the  constituents  themselves  may  be  those 
born  at  the  new  temperature. 


FIG.  39.— Frame  Steel  Partly  Annealed.     X60.    (Bullens.) 

This  point  is  illustrated  in  Figs.  38,  39  and  40.  These  are 
photomicrographs  taken  from  tests  made  upon  chrome  nickel  steel 
plates  for  automobile  frames :  Fig.  38  shows  the  structure  of  the  steel 


50  STEEL  AND   ITS  HEAT  TREATMENT 

as  rolled;  Fig.  39  shows  the  steel  after  a  short  annealing  at  a  tem- 
perature above  the  upper  critical  range;  and  Fig.  40  shows  the  same 
steel  after  a  long  anneal  at  the  same  temperature.  It  will  be  noticed 
that  the  steel  in  Fig.  39  has  taken  on  approximately  the  same  struc- 
tural constituents  as  in  the  fully  annealed  piece  as  shown  in  Fig.  40, 
but  that  it  still  remains  in  the  "stressed  condition  of  Fig.  38,  even 
though  the  annealing  temperatures  were  the  same  in  both  cases. 
It  is  important,  therefore,  if  a  soft  steel,  free  from  all  internal  strains 
and  stresses  is  desired,  that  a  sufficient  length  of  time  be  allowed  for 
the  permanent  elimination  of  these  intermolecular  strains,  before  and 
after  cooling.  In  the  case  of  the  steel  plate  just  referred  to  it  re- 


FIG.  40.— Frame  Steel  Fully  Annealed.     X60.     (Bullens.) 

quired  some  twelve  hours  for  the  complete  change  or  equalization  to 
take  place! 

"  Milky-Ways." — We  have  previously  explained  this  same 
phenomenon  under  the  heading  of  "  Diffusion,"  as  this  is  the  scientific 
principle  underlying  it.  The  reoccurrence  or  reformation  of  these 
laminations  or  other  stressed  structures  is  due  to  the  fact  that  the 
complete  effacement  by  equalization  had  not  taken  place.  In  other 
words,  it  means  that  where  these  stressed  areas  occur  the  carbon 
content  as  a  whole  is  less  than  in  the  rest  of  the  mass.  Where  ferrite 
predominates,  as  in  the  lower  carbon  steels,  there  will  the  mass  more 
easily  coalesce  into  what  may  be  termed  "  milky-ways  "  (Howe). 
In  order  to  equalize  the  steel  as  a  whole  the  length  of  time  of  the 
sojourn  at  or  slightly  above  Ac3  should  be  inversely  proportional 
to  the  time  occupied  in  reaching  that  temperature. 


ANNEALING  51 

Alloy  Steel. — Alloy  steel  is  particularly  an  example  of  the  re- 
tarded transformation  as  described  above,  although  the  author  has 
repeatedly  found  it  in  carbon  steels  cold  worked.  Most  notable  of 
the  alloy  steels  exhibiting  this  peculiarity  are  chrome,  chrome- 
nickel  and  chrome-vanadium  steels.  Many  users  and  even  manu- 
facturers of  these  steels  contend  that  annealing  will  not  give  entirely 
satisfactory  results.  The  oft-encountered  "  hard-spots "  would 
seem  to  bear  out  this  dispute.  From  the  experience  of  the  author 
the  proposition  develops  into  a  simple  question  of  time.  The  alloy- 
ing metals  add  to  the  density  of  the  grain,  so  that  a  longer  time  is 
needed  to  complete  the  change  in  entirety.  It  was  found  that  a 
certain  3-inch  rolled-round  approximating  0.50  per  cent,  carbon, 
1.50  per  cent,  nickel  and  0.50  per  cent,  chrome  required  sixteen  hours 
for  this  complete  change,  together  with  the  elimination  of  hard- 
spots,  to  take  place ;  high-carbon  high-chrome  steel  often  takes  days 
for  a  complete  anneal. 

Time  of  Heating. — Assuming  a  proper  rate  of  heating,  it  therefore 
remains  to  determine  the  required  length  of  heating  by  means  of 
experimentation,  taking  into  consideration  such  points  as  have  been 
mentioned  above.  Along  these  lines  some  interesting  experiments 
have  been  carried  out  by  Mr.  M.  E.  Leeds  l  for  the  determination 
of  the  variations  in  rates  of  heating  of  specimens  of  different  sizes 
to  various  furnace  temperatures  and  which  in  some  degree  answer 
the  oft- repeated  question  "  How  long  shall  we  heat  this  piece  of 
steel?  "  The  experiments  were  made  with  round  specimens  of  nor- 
mal open-hearth  carbon  steel  approximating  0.5  per  cent,  carbon, 
and  ranging  in  size  from  2  ins.  to  12  ins.  in  diameter,  by  24  ins.  long. 
Each  specimen  was  heated  to  four  temperatures,  namely,  1000°, 
1200°,  1400°,  and  1600°  F.  During  the  time  of  heating  a  continuous 
record  was  kept  of  furnace  temperatures,  the  temperature  of  the  sur- 
face of  the  specimen,  and  of  one  to  three  points  in  its  interior.  While 
the  results  obtained  are  necessarily  of  relative  value  only  on  account 
of  the  varying  furnace  conditions  which  might  be  found  elsewhere, 
there  are,  nevertheless,  several  interesting  conclusions  of  value 
which  were  drawn  from  these  experiments: 

1.  "Variation  in  Time  of  Heating  with  Size. — As  would  be 
expected,  the  smaller  specimens  heat  more  rapidly  than  the  larger. 
In  curves  (Fig.  41)  the  relation  between  the  size  of  specimen  and 
time  of  heating  to  various  temperatures  are  brought  out.  Except 
in  a  very  general  way,  this  information  could  not  be  used  as  a  guide 
1  M.  E,  Leeds,  A.  S.  T.  M.,  June  Meeting,  1915. 


52 


STEEL  AND   ITS   HEAT   TREATMENT 


to  heating  practice,  as  the  rates  would  vary  with  the  size  of  furnace 
and  probably  with  other  conditions. 

2.  "  Relation  between  Time  of  Heating  and  Furnace  Tempera- 
ture.— The  time  of  heating  for  a  specimen  of  any  size  is  less  when  it 
is  brought  up  to  1600°  F. than  when  brought  up  to  1200°  F.,  and  less 


I5 

o 

I 


300 


X 


/ 


4" 


8" 
Size  of  Section. 


FIG.  41.    Curves  Derived  from  Rate  of  Heating. 

thrup  Co.) 


(Courtesy  of  Leeds  &  Nor- 


F. 


for  1200°  F.  than  for  1000°  F.,  although  it  is  greater  for  1400 
than  for  any  other  temperature. 

"  It  is  more  difficult  to  account  for  the  fact  that  the  higher 
temperatures  are  attained  more  rapidly  than  the  lower  ones.  This 
fact,  however,  appears  to  be  clearly  demonstrated.  It  may  be  that 


ANNEALING  53 

the  specimens  received  a  large  amount  of  their  heat  by  radiation  from 
the  furnace  walls.  The  heat  transfer  by  radiation  between  two  bodies 
at  different  temperatures  is  proportional  to  the  difference  between  the 
fourth  powers  of  their  absolute  temperatures,  and  so  for  a  100°  dif- 
ference in  temperature  between  furnace  wall  and  test  specimens, 
at  1600°  F.,  the  heat  transfer  would  be  at  a  higher  rate  than  for  the 
same  temperature  difference  at  lower  temperatures. 

3.  "  Relation  between   Surface   and  Interior  Temperatures.— 
From  all  of  the  curves,  it  is  deduced  that  there  is  no  large  difference 
in  temperature  of  the  points  inside  of  the  specimen.     This  was  quite 
surprising,  as  it  was  expected  that  the  12-in.  specimen  would  show 
considerable  differences  of  temperature  between  a  point  2  ins.  from 
the  surface  and  the  center. 

"  All  of  the  runs  show  that  the  contact  couple  is  at  a  higher 
temperature  than  any  of  the  interior  couples  until  the  specimen 
has  attained  the  temperature  of  the  furnace.  It  cannot  properly  be 
assumed  that  the  temperature  shown  by  the  contact  couple  is  exactly 
that  of  the  surface  of  the  specimen. 

"  When  the  contact  couple  attains  the  furnace  temperature,  all 
parts  of  the  specimen  have  also  attained  that  temperature.  This 
suggests  a  practical  method  of  using  contact  couples  in  conjunction' 
with  furnace  couples,  namely,  by  means  of  the  furnace  couple  the 
furnace  should  be  held  at  the  temperature  at  which  it  is  desired  to 
treat  the  specimen,  and  the  contact  couple  should  then  be  used  to 
determine  when  the  specimen  has  assumed  the  desired  temperature. 

4.  "  Contact    Couple    Shows    Time    of    Transformation. — The 
curves  (not  given  here)  showing  the  heating  of  the  12-in.  and  8-in. 
specimens  to  1400°  and  1600°  F.  show  that  the  transformation  point 
is  clearly  shown  by  the  couples  inside  of  the  specimen,  and  that  it 
is  also  shown  by  the  contact  couple.     The  interior  couples  show, 
with  approximate  correctness,  the  temperature  at  which  the  trans- 
formation takes  place.     The  contact  couple  shows  a  corresponding 
flexure  in  its  curvature,  at  the  same  time  as  the  interior  couples, 
though  not  at  the  same  temperature."     The  close  correspondence 
in  time  between  the  flexures  of  the  contact  couple  and  the  interior 
couples   points  to  what  Mr.  Leeds  believes  is  an  important  new 
method  of    determining  when   a  piece  of   steel    has   been    heated 
through  its  transformation  point. 

Rate  of  Cooling. — We  know  from  our  study  of  the  previous 
chapters  that  in  hypo-eutectoid  steels  the  solid  solution  rejects  the 
excess  ferrite  upon  cooling  through  the  transformation  range.  This 


54  STEEL  AND   ITS  HEAT  TREATMENT 

ferrite  will  form  either  a  network  around  the  grains  of  solid  solution 
or  pearlite,  or  will  coalesce  into  irregular  masses,  the  same  being 
dependent  upon  the  rate  of  cooling.  A  moderately  slow  cooling  will 
develop  the  cellular  or  network  structure  without  breaking  it  up. 
A  very  slow  cooling  will  break  up  the  network  structure,  giving 
ample  time  for  the  ferrite  to  coalesce  into  large  masses.  The  slower 
the  cooling  through  the  transformation  ranges,  the  greater  also  will 
be  the  size  of  the  grains. 

Effect  of  Cooling. — The  effect  of  the  varied  rate  of  cooling  is 
illustrated  in  the  photomicrographs  of  a  0.45  per  cent,  carbon  steel 


FIG.  42. — Network  Structure,  0.45  per  cent.  Carbon  Steel.     X100.     (Bullens.) 

shown  in  Figs.  42,  43  and  44,  all  taken  at  the  same  magnification. 
All  three  pieces  were  heated  to  a  temperature  somewhat  in  excess  of 
the  full  annealing  temperature.  The  steel  of  Fig.  42  was  cooled  quite 
rapidly  (air-cooled) ;  that  of  Fig.  43  was  cooled  rapidly  through  the 
upper  part  of  the  transformation  range,  but  slowly  through  the  lower 
critical  range ;  that  of  Fig.  44  was  cooled  with  the  furnace.  Thus  we 
have  the  network  structure  in  the  first  case,  showing  a  comparatively 
small  grain-size.  In  the  second  instance  the  network  is  coarse 
and  the  pearlite  is  fairly  well  developed.  A  very  slow  cooling,  as  in 
the  third  case,  has  resulted  in  a  coalescence  of  the  ferrite  into  large 
grains,  intermingling  with  the  coarse  pearlite.  The  ferrite  in  all 
three  photomicrographs  is  represented  by  the  white  constituent. 


ANNEALING 


55 


Some  very  interesting  facts  might  be  drawn  from  a  study  of  these 
photomicrographs  in  comparison  with  those  previously  mentioned 
in  the  series  of  Figs.  27  to  31,  and  Figs.  32  to  36. 


FIG.  43. — Coarse  Network  Structure,  0.45  per  cent.  Carbon  Steel. 

(Bullens.) 


X100. 


FIG.  44. — Coalesced  Ferrite  and  Pearlite,  0.45  per  cent.  Carbon  Steel. 

(Bullens.) 


xioo. 


56 


STEEL  AND  ITS  HEAT  TREATMENT 


Effect  of  the  Rate  of  Cooling  upon  the  Pearlite. — Not  only  does 
the  rate  of  cooling  from  the  annealing  temperature  have  a  very  great 
effect  upon  the  network  and  grain  structure,  but  also  upon  the  char- 
acteristics of  the  pearlitic  constituents  of  the  steel.  The  rate  of 


MlCROSTRUCTURE. 


SEGREGATION  STAGES. 


I.  Sorbite  or  "  sorbitic  pearl- 
ite."  Cementite  emulsi- 
fied. 


MECHANICAL  PROPERTIES. 


Tensile  strength  about  150,000 

Ibs.  per  sq.  in. 
Elongation  about  10%  in  2  ins. 


II.  Sorbite   passing  into    nor-     Tensile  strength  about  125,000 
malpearlite.  Semi-segre-          Ibs.  per  sq.  in. 
gated  cementite.  Elongation  about  15%  in  2  ins. 


III.  Finely  laminated  pearlite. 


IV.  Laminated  pearlite.  Com- 
pletely segregated  ce- 
mentite. 


Tensile  strength  about  100,000 

Ibs.  per  sq.  in. 
Elongation  about  10%  in  2  ina. 


Tensile  strength  about  85,000 

Ibs.  per  sq.  In. 
Elongation  about  8%  in  2  ina. 


Cementite — white 
Ferrite — black. 


V.  Laminated   pearlite    pass-  Tensile    strength    about    75,000 

ing    into  massive    pearl-  Ibs.  per  sq.  in. 

i  t  e  .       Cementite     and  Elongation    about  5%    in  2  ins. 
ferrite    each    coagulating 


FIG.  45. — Pearlitic  Development. 


cooling  through  the  lower  critical  range,  at  which  the  transformation 
of  the  solid  solution  into  pearlite  is  effected,  will  so  change  the 
arrangement  of  the  ferrite  and  cementite  constituents  of  the  pearlite 
that  widely  varying  physical  results  may  be  obtained  in  this  manner. 


ANNEALING  57 

As  we  will  explain  later,  the  austenUe  does  not  directly  change  into 
pearlite,  but  passes  through  a  series  of  transition  constituents  with 
varying  physical  properties.  The  majority  of  these,  however,  are 
not  retained  in  the  steel  through  methods  of  cooling  other  than 
quenching  (which  may  or  not  be  followed  by  a  reheating),  so  that 
we  need  consider  only  the  very  last  transition,  sorbite.  This  com- 
ponent sorbite  represents  the  last  stage  of  the  transition  austenite  to 
pearlite,  and  in  which  the  individual  particles  of  ferrite  and  cementite 
are  just  on  the  verge  of  coalescing.  Sorbite,  or  sorbitic-pearlite, 
is  noted  for  its  combination  of  high  tensile  strength  (i.e.,  in  comparison 
with  the  later  phases  of  pearlite)  and  ductility.  Sorbite  is  generally 
formed  by  air  cooling  through  the  lower  critical  range,  and  is  shown 
in  Fig.  45.  This  figure  also  illustrates  the  different  phases  of  the 
pearlite,  together  with  their  approximate  physical  characteristics. 
From  this  it  will  be  evident  that  the  rate  of  cooling  must  necessarily 
have  a  great  influence  upon  the  physical  properties  of  the  slowly 
cooled  or  annealed  steel,  and  that  the  operation  must  be  adjusted 
accordingly. 

Definite  Cooling. — Thus  we  see  that  having  obtained  the  per- 
manent release  of  all  internal  strains  and  stresses  and  brought  about 
the  formation  of  an  entirely  new  grain-size  and  structure  by  means  of 
proper  heating,  it  now  remains  to  adjust  the  physical  properties  by 
means  of  regulating  the  rate  of  cooling.  In  general,  there  are  three 
methods  of  cooling  as  used  in  the  annealing  process.  These  are: 
(1)  Cooling  in  and  with  the  furnace,  (2)  removing  the  steel  from 
the  furnace  and  covering  with  some  blanketing  substance  such  as 
lime,  sand,  ashes,  etc.,  (3)  cooling  in  air.  Cooling  by  means  of 
quenching  is  not  a  true  annealing  operation,  and  will  therefore  be 
considered  under  the  subject  of  "  Hardening." 

Furnace  Cooling. — Cooling  in  and  with  the  furnace  will  generally 
give  the  slowest  cooling  of  the  steel  which  is  possible  if  the  furnace 
is  of  heavy  construction  and  can  be  tightly  closed.  Furnace  cooling 
will  give  a  maximum  "  softness  "  and  ductility — that  is,  the  tensile 
strength  and  elastic  limit  will  be  at  a  minimum,  and  the  elongation 
and  reduction  of  area  will  be  large.  Steel  in  this  condition  will  be 
in  a  suitable  condition  for  ordinary  machining,  and  will  also  have 
the  quality  of  resisting  a  small  number  of  severe  distortions. 

Slow  Cooling. — In  cases  where  the  objects  are  of  large  size,  an 
approximation  of  furnace  cooling  may  be  obtained  by  removing  the 
steel  from  the  furnace  and  covering  with  some  blanketing  substance 
and  slow  conductor  of  heat  such  as  lime,  sand  or  ashes.  This  will 


58  STEEL  AND  ITS  HEAT  TREATMENT 

also  permit  the  recharging  of  the  furnace  for  another  heat  without 
loss  of  time. 

Pit  Annealing. — Where  a  large  tonnage  of  steel  must  be  annealed, 
a  pit  lined  with  brick  or  concrete  and  suitably  fitted  with  cover 
plates  is  sometimes  made.  The  hot  steel  is  immediately  delivered 
from  the  annealing  furance  to  the  pit  and  covered  with  ashes.  Cool- 
ing by  this  method  of  pit-annealing  is  often  slower  than  cooling  in 
the  furnace  itself  if  the  latter  is  not  properly  constructed  so  that  no 
cold  air  can  find  its  way  in. 

Size,  of  Object. — It  is  readily  realized  that  the  size  of  the  object 
has  a  great  bearing  upon  the  rate  of  cooling.  Under  the  same  con- 
ditions a  smaller  object  will  cool  much  more  rapidly  and  will  there- 
fore be  harder  and  less  ductile  than  a  piece  of  considerable  size. 
The  rate  of  cooling  must  therefore  be  proportioned  to  the  size  of 
the  object. 

Air  Cooling. — If  the  steel  is  removed  from  the  furnace  and  allowed 
to  cool  in  air  the  physical  properties  will  be  proportional  to  the 
dimensions  of  the  piece  and  also  dependent  upon  the  carbon  content. 
Thin  objects  and  those  with  high  carbon  content  cannot  stand  so 
rapid  a  cooling  as  thick  and  low  carbon  ones,  lest  their  ductility  be 
too  greatly  sacrificed.  In  this  regard  the  American  Society  for 
Testing  Materials  recommends  the  following:  "  Thick  objects  with 
less  than  0.50  per  cent,  of  carbon  may  be  cooled  completely  in  air, 
of  course  protected  from  rain  or  snow.  Objects  with  0.50  per  cent, 
of  carbon  or  more,  and  thin  objects  with  from  0.30  to  0.50  per  cent, 
of  carbon  may  be  cooled  in  air  if  their  cooling  is  somewhat  retarded, 
as,  for  instance,  by  massing  them  together,  as  happens  in  the  case 
of  rails."  This  more  rapid  cooling  will  give  great  strength  and 
high  elastic  limit,  but  less  ductility. 

Combination  Air  and  Furnace  Cooling. — Besides  the  regular  air 
or  furnace  cooling  there  are  a  number  of  different  combinations  of  the 
two  which  have  given  great  success  in  innumerable  cases.  We  will 
give  them  briefly  as  follows: 

1.  Heat  to  slightly  over  Ac3,  air  cool  to  just  over  Arl,  return  to 

a  furnace  which  is  held  at  that  temperature  (about  1350°  F.),  heat 

until  uniform,  and  then  cool  slowly.     The  latter  heating  should  not 

be  any  longer  than  is  possible.     This  method  will  tend  to  prevent  the 

formation  of  large  amounts  of  free  ferrite,  but  will  affect  the  pearlite, 

as  there  will  be  slow  cooling  through  the  Arl  range. 

(f*1*^.  Heat  to  slightly  over  the  Ac3  range,  air  cool  to  just  under  the 

i  Arl  range,  return  to  a  furnace  and  heat  at  1350°  F.  and  slow  cool. 


ANNEALING  59 

This  method  will  effect  a  greater  "  toughening  "  if  the  temperature 
has  not  been  prolonged  too  greatly  at  the  second  heating. 
&  3.  Heat  to  slightly  above  Ac3,  air  cool  to  below  Arl,  return  to  a 
furnace  heated  at  a  temperature  slightly  below  Arl  (about  1200°  to 
1250°  F.),  hold  at  this  temperature  until  uniformly  heated,  and  slow 
cool.  In  fact,  the  last  cooling  may  be  made  in  the  air  if  desired,  as 
there  will  be  little  or  no  change  in  cooling  from  under  the  lower 
Critical  range. 

Fine  Grain  Annealing. — Of  the  three  special  methods  given,  the 
third  is  the  preferable,  as  well  as  the  most  uniform  and  certain  in 
results.  By  permitting  the  steel  to  air  cool  to  a  temperature  below 
the  lowest  transformation,  advantage  is  taken  of  any  "  hardening 
effect  "  or  retardation  in  the  transformation  of  austenite  into  a 
conglomerate  of  pearlite  and  ferrite.  This  effect  will  increase 
with  the  percentage  of  carbon  and  the  smaller  the  size  of  the  piece. 
The  reheating  to  a  temperature  below  the  lower  critical  range,  if 
not  prolonged,  will  neither  change  the  grain  size  nor  allow,  of  the 
coalescing  of  the  excess  ferrite  or  of  the  individual  constituents  of 
the  pearlite,  but  will  form  a  mass  of  irresolvable  and  intermixed 
pearlite  and  ferrite  known  as  "  sorbite."  At  the  same  time,  however, 
it  will  give  the  maximum  combination  of  large  ductility,  good  strength 
and  excellent  machining  properties.  This  method  is  of  particular 
value  in  the  annealing  of  tool  steels,  in  which  it  has  given  most 
excellent  results. 

The  main  objection  to  both  of  the  other  two  methods  is  that  a 
varying  duration  of  heating  above  the  lower  critical  range  will  cause 
corresponding  changes  in  the  results,  so  that  no  absolutely  definite 
result  capable  of  commercial  duplication  can  be  obtained.  The 
methods,  however,  find  many  applications  in  a  general  way,  particu- 
larly in  steels  of  medium  carbon  content  which  have  been  severely 
stressed  by  previous  mechanical  elaboration.  The  second  method 
especially  will  give  the  advantage  of  having  had  a  double  heating 
through  the  lower  critical  range  (besides  the  minimum  grain-size 
conferred  by  fairly  rapid  cooling  from  the  upper  critical  range),  and 
thus  breaking  up  the  previous  structure. 

Double  Annealing. — The  next  variable  which  may  be  used  is 
that  of  heating  to  a  temperature  considerably  in  excess  of  the  upper 
critical  range,  air  cooling  to  under  the  lower  critical  range,  and 
reheating  to  slightly  above  the  lower  or  upper  critical  range.  As; 
an  example  of  this  the  author  will  cite  a  case  which  was  success- 
fully solved  by  this  method.  Certain  medium-carbon  steel  plates 


60  STEEL  AND  ITS  HEAT  TREATMENT 

had  been  finished  at  a  temperature  considerably  under  the  proper 
temperature  for  hot-rolling  and  thus  had  been  considerably  stressed 
— in  fact,  the  ordinary  annealing  method  would  not  relieve  this  con- 
dition. The  plates  were  put  in  a  furnace  with  a  car-bottom,  heated 
thoroughly  at  about  1700°  F.  (that  is,  considerably  over  the  Ac3 
range),  air  cooled  until  black,  and  then  reheated  and  slow  cooled 
from  a  temperature  slightly  over  the  lower  critical  range.  The 
laminations  occasioned  by  the  rolling  were  entirely  eliminated  by 
the  high  temperature,  and  their  reformation  prevented  by  the  rapid 
cooling  in  air.  The  second  anneal  then  thoroughly  softened  the 
steel  and  put  it  in  good  condition  for  the  following  forming  opera- 
tions. This  steel  might,  of  course,  have  been  reannealed  at  the 
Ac3  range  (instead  of  the  Acl  range)  and  an  even  better  product 
obtained. 

Tool  Steel  Annealing. — The  annealing  of  hypo-eutectoid  tool 
steel  may  be  broadly  grouped  under  two  headings,  dependent  upon 
the  initial  condition  of  the  steel  and  upon  the  results  desired.  Tool 
steel  which  has  been  carefully  hammered  is  undoubtedly  strength- 
ened by  this  mechanical  elaboration;  a  full  annealing — that  is,  heat- 
ing at  a  temperature  over  the  critical  range — will  entirely  destroy 
the  results  of  the  forging  operation.  If  it  is  therefore  desired  simply 
to  anneal  the  steel  in  order  to  put  it  in  suitable  condition  for  machine 
work — that  is,  to  soften  it  and  at  the  same  time  to  retain  the  bene- 
ficial effects  of  the  forging — the  annealing  operation  should  be 
carried  out  at  a  temperature  less  than  that  of  the  critical  range,  or  in 
the  neighborhood  of  1200°  to  1250°  F.  On  the  other  hand,  if  it 
is  desired  to  obtain  the  finest  grain  size  possible,  the  maximum 
softness,  and  to  entirely  eliminate  any  previous  heating  or  forging 
work,  the  annealing  should  be  carried  out  at  a  temperature  slightly 
over  that  of  the  critical  range,  or  in  the  neighborhood  of  1400°  F., 
dependent  upon  the  composition  of  the  steel  in  question. 
k*.  Protection  of  Steel. — One  of  the  vital  points  in  obtaining  a  satis- 
factory steel  after  annealing  is  the  protection  of  its  surface.  Steel 
when  heated  beyond  a  low-red  heat  exhibits  a  great  tendency  to 
oxidize  or  scale,  this  action  increasing,  in  the  presence  of  oxygen, 
with  the  temperature  and  the  length  of  time  involved.  This  con- 
dition will  exist  in  furnaces  operated  so  as  to  produce  sharp  heats, 
instead  of  soft,  slightly  hazy,  reducing  atmospheres.  Decarburiza- 
tion  to  a  depth  of  J  to  \  inch  is  not  a  rare  occurrence  where  improper 
combustion  and  heat  application  is  the  rule.  If,  due  to  poor  furnace 
design  and  worse  operation,  such  conditions  do  exist  • 


ANNEALING  61 

produce  a  clean  surface  it  will  be  necessary  to  protect  the  exposed 
surface  of  the  steel  in  some  manner.  Tool  steel  is  often  annealed 
by  placing  in  a  tube,  packing  carefully  with  charcoal,  and  then  clos- 
ing the  ends  of  the  tube  with  caps  or  luting  with  clay. 

On  the  other  hand,  the  prevention  of  oxidation  or  the  scaling  of 
the  metal  during  the  heating  process  is  a  simple  thing  with  the 
proper  furnace  design  and  operation.  Assuming  such  a  design,  if  the 
furnace  is  operated  so  as  to  produce  soft,  hazy  heats  such  as  we  have 
previously  mentioned,  there  should  be  no  occasion  for  packing  the 
steel  in  charcoal  or  other  such  substances.  This  statement  is  made 
not  as  one  of  theory,  but  as  one  of  actual  practice.  Under-fired  fur- 
naces are  being  run  to-day  on  brass  cartridge  case  work  in  which  there 
is  less  oxidation  and  decolorization  of  the  metal  than  in  other  fur- 
naces in  which  the  metal  is  packed  in  charcoal;  not  only  is  a  better 
product  being  obtained,  but  at  less  operating  copt.  \~  „ 

Box-Annealing. — For  the  protection  of  larger  masses  or  a  number 
of  smaller  pieces,  "  box-annealing  "  is  often  resorted  to.  This  par- 
ticularly applies  to  cases  where  a  finished  surface  must  not  be 
injured.  The  steel  is  placed  in  a  rectangular  pot  or  box  made  of 
cast  iron  or  of  plates  riveted  together.  This  box  may  or  not  be 
lined  with  some  refractory  substance  such  as  silica  brick.  The  metal 
is  then  carefully  packed  with  some  material  such  as  ground  mica, 
sand,  charcoal,  charred  bone  or  leather,  lime,  etc.  If  the  steel  is 
low  in  carbon  a  carbonaceous  or  carbon  monoxide  generating  sub- 
stance must  not  be  used,  for  a  slight  case-hardening  action  would 
take  place.  In  the  case  of  higher  carbon  steels,  and  especially  of 
tool  steels,  reducing  agents  may  be  used,  although  it  is  better  to  mix 
the  charcoal  with  clean  ashes.  Sand  and  ground  mica  are  probably 
the  most  satisfactory  of  the  simple  non-reducing,  refractory  materials. 
The  cover  is  then  placed  on  the  box  and'  the  box  with  its  contents  is 
charged  into  the  furnace  and  given  the  proper  degree  and  duration  of 
heating..  The  box  should  be  raised  from  the  floor  of  the  furnace  so 
that  the  hot  gases  may  have  opportunity  for  circulation  around  it. 
When  properly  heated  throughout,  the  box  may  be  removed  from  the 
furnace  and  allowed  to  cool  to  atmospheric  temperature. 

Stead's  Brittleness. — We  have  previously  stated  that  practically 
no  change  occurs  below  the  Acl  range  if  no  previous  hardening  of  the 
steel  has  taken  place.  The  one  exception  is  that  of  very  low-carbon 
steels  and  is  due  to  the  fact  that  steels  very  low  in  carbon  behave 
more  like  pure  or  carbonless  iron,  there  being  but  small  percentages 
of  cementite  (and  therefore  pearlite)  to  influence  the  grain-size. 


62  STEEL  AND   ITS  HEAT  TREATMENT 

Upon  heating  such  steels  through  the  upper  part  of  zone  la  (refer 
to  diagram  in  Fig.  13),  a  distinct  coarsening  of  the  ferrite  grains 
occurs,  this  being  a  function  of  time  as  well  as  of  temperature. 
Steels  of  such  carbon  held  at  say  1100°  F.  for  a  considerable  length  of 
time  will  develop  such  coarsening  of  grain-size  as  to  make  the  steel 
unfit  for  commercial  use  if  any  degree  of  strength  is  required.  This 
phenomenon  is  known  as  "  Stead's  Brittleness."  With  steels  of 
greater  carbon  content  the  increased  pearlite  so  operates  upon  the 
molecular  structure  of  the  steel  that  practically  no  change  occurs 
until  the  Acl  range  is  reached. 

ANNEALING   HYPER-EUTECTOID    STEELS 

Critical  Ranges. — Strictly  speaking,  hyper-eutectoid  steels  have 
two  critical  ranges:  the  A  1.2.3,  at  which — on  heating — the  pearlite 
changes  into  the  solid  solution;  and  the  A  cm  range,  at  which — on 
heating — there  is  the  final  solution  of  the  excess  cementite— just  as 
in  hypo-eutectoid  steels  the  Ac3  range  represents  the  solution  of  the 
last  of  the  excess  ferrite.  However,  on  account  of  the  relatively 
small  proportion  of  free  cementite  in  the  ordinary  hyper-eutectoid 
steels,  and  also  because  there  is  a  large  increase  in  grain-size  upon 
heating  to  the  Ac. cm  range — the  temperature  position  of  the  latter 
increasing  very  rapidly  with  increase  in  the  carbon  content — the 
Ac. cm  range  requires  .but  little  practical  consideration  and  the 
majority  of  the  annealing  operations  are  more  intimately  connected 
with  the  principal  critical  range  Ac  1.2.3. 

Commercial  Annealing. — Similarly  to  hypo-eutectoid  steels,  the 
annealing  of  high-carbon  steels  may  have  for  its  object  any  or  all  of 
the  following  factors:  (1)  the  release  of  internal  strains  and  stresses 
set  up  by  previous  operations,  (2)  the  softening  of  the  steel  to  place 
it  in  a  suitable  condition  for  machining,  (3)  the  entire  change  of 
structure. 

The  first  item  may  be  accomplished  by  a  simple  reheating  at 
temperatures  below  those  of  the  critical  range.  The  second  and 
third  items  are  more  complex  in  their  solution,  as  the  form  in  which 
the  excess  cementite  may  exist  is  one  of  the  governing  factors. 

If  the  mass  of  the  steel  is  in  the  sorbitic  state,  as  may  generally 
be  expected  in  the  usual  tool  steel,  satisfactory  results  (the  softening 
of  the  steel  for  machining,  and  relieving  the  internal  strains)  may  be 
obtained  by  an  annealing  at  a  temperature  slightly  under  that  of  the 
principal  critical  range,  or  at  about  1250°  to  1300°  F.  This  heating 


ANNEALING  63 

should  not  be  prolonged  for  such  length  of  time  as  may  cause  the 
excess  cementite  to  coagulate,  but  only  until  the  steel  has  been  thor- 
oughly and  uniformly  heated  throughout. 

On  the  other  hand,  if  it  is  desired  to  obtain  the  complete  change 
of  structure,  and  to  refine  the  grain  (previously  coarse),  it  will  be 
necessary  to  heat  to  a  temperature  at  least  in  excess  of  the  Ac  1.2.3 
range  (about  1340°  F.).  For  steels  with  a  carbon  content  approx- 
imating 0.9  per  cent.,  such  heating  will  accomplish  the  complete 
change  of  structure  and  give  the  finest  grain-size  obtainable  through 
annealing.  For  steels  with  a  carbon  content  considerably  in  excess 
of  the  eutectoid  ratio  the  annealing  may  be  done  at  similar  tempera- 
tures, provided,  however,  that  the  excess  cementite  is  more  or  less 
in  solution  in  the  sorbite. 

Incidentally,  if  the  condition  stated  under  (3)  is  desired,  and  it 
will  warrant  the  expense,  the  best  method  is  first  to  oil  quench  from 
a  temperature  somewhat  over  the  Ac  1 .2.3  range,  and  subsequently 
anneal  at  a  temperature  just  below  that  range. 

Normalizing. — If  the  steel  to  be  annealed  has  the  free  cementite 
existing  as  network  or  spines,  which  would  make  the  steel  difficult  of 
machining,  annealing  at  the  usual  temperatures  (Ac  1.2.3)  will  not 
affect  this  cementite:  it  will  simply  refine  the  ground-mass.  In 
order  to  eliminate  this  free  cementite,  it  will  be  necessary  first  to 
normalize  or  quench  the  steel  from  a  temperature  above  that  of  the 
Ac. cm  range.  That  is,  air  cooling  from  a  temperature  of  say  1750° 
or  1800°  F.  will  not  permit  of  the  reformation  of  coagulated  cement- 
ite. The  second  annealing  may  then  be  carried  out  at  a  temperature 
of  1375°  with  the  refining  of  the  grain  size  and  complete  softening  of 
the  steel  as  a  whole;  this  second  heating  should  be  just  as  short 
as  possible  in  order  to  prevent  the  reformation  of  the  free 
cementite. 

Spheroidizing  the  Cementite. — The  above  method  may  be  further 
modified  by  reheating  to  a  temperature  slightly  under  the  lower 
critical  range  instead  of  over  it.  The  objection  to  this  is  that  the 
steel  will  not  be  refined,  but  will  possess  the  large  grain  size  charac- 
teristic of  the  high  temperature.  On  the  other  hand,  the  lower 
annealing  temperature  will  entirely  prevent  the  formation  of  the  free 
cementite  as  either  spines  or  as  a  network.  Instead,  it  will  be  found 
that  the  excess  cementite  will  be  thrown  out,  under  these  conditions, 
as  little  nodules  or  "  spheroids  "  if  the  reheating  temperature  is  just 
about  at  the  end  of  the  lower  critical  range;  or,  under  certain  con- 
ditions, the  whole  mass  of  the  steel  may  be  called  "  granular,"  if  such 


64  STEEL  AND   ITS  HEAT  TREATMENT 

/ 

a  term  is  permissible.  Further  reference  to  this  spheroidal  forma- 
tion of  cementite,  as  obtained  by  a  double  "  quenching,"  is  given 
under  Chapter  VII.  Spheroidal  cementite  in  annealed  steels  may 
also  be  obtained  by  very  slow  cooling  through  the  end  of  the  Arl 
transformation:  cementite  in  this  condition  is  a  great  help  in  the 
machining  of  high-carbon  steels. 


CHAPTER   IV 
HARDENING 

Hardening. — Fundamentally,  the  operation  of  hardening  in- 
volves two  operations  of  change  in  temperature :  heating  and  cooling. 
The  function  of  the  heating  is  (1)  to  obtain  the  best  refinement,  and 
(2)  to  obtain  the  formation  of  the  "  hard  "  constituents  of  the  steel. 
Having  done  this,  the  steel  must  then  be  held  in  this  condition  by 
very  rapid  cooling — that  is,  by  quenching  in  some  medium  such  as 
water  or  oil.  Associated  with  both  the  heating  and  rapid  cooling 
there  must  be  as  great  a  degree  of  uniformity  as  is  possible. 

Changes  on  Heating. — Steel,  when  properly  hardened,  should 
show  no  trace  of  the  original  structure,  such  as  coarse  grain  size, 
network,  unabsorbed  ferrite  (in  hypo-eutectoid  steels),  or  any  other 
peculiarities  of  untreated  steel.  If  such  are  present  in  the  hardened 
steel  it  goes  to  prove  that  the  operation  was  not  properly  carried  out. 
Further,  if  the  structure  of  the  steel  has  not  been  suitably  changed  or 
developed  by  the  heating  operation,  it  most  assuredly  will  not  be 
altered  for  the  better  by  subsequent  quenching.  The.most  that  such 
quenching  can  do  is  to  retain  the  characteristics  which  the  heating 
has  developed. 

An  attempt  has  been  made  graphically  to  illustrate  these  facts 
in  the  chart  in  Fig.  46.  Column  1  (at  the  left)  represents  a  normal, 
0.4  per  cent,  carbon,  pearlitic  steel  (at  the  bottom  of  the  column), 
and  the  structural  changes  taking  place  in  that  steel  as  it  is  pro- 
gressively heated  through  and  beyond  the  critical  ranges.  For  the 
present  ,it  is  assumed  that  the  structure  and  micrographic  constit- 
uents obtained  by  heating  to  various  temperatures,  such  as  A  to  E, 
may  be  retained  by  quenching,  as  illustrated  by  columns  II  to  VI. 

Thus  heating  to  a  temperature  A,  under  that  of  the  lower  critical 
range,  will  produce  no  change  in  the  original  steel,  which  consists  of 
pearlite  (the  cross-hatched  circles)  and  ferrite  (the  black  area). 
The  quenching  likewise  will  produce  no  change,  as  is  illustrated  by 
Column  II. 

Heating  to  a  temperature  B,  slightly  over  the  lower  critical  range, 
will  change  the  pearlite  to  the  solid  solution  (represented  by 

65 


66 


STEEL  AND   ITS  HEAT  TREATMENT 


the  dotted  area),  but  without  affecting  the  free  ferrite.  Quenching, 
column  III,  will  therefore  produce  a  semi-hardened  steel — since  the 
solid  solution  is  the  "  hard  "  constituent — with  a  refinement  of  the 
"  ground-mass  "  (the  original  pearlite)  only. 

Heating  to  a  temperature  C,  between  the  lower  and  upper  critical 
ranges,  will  effect  a  progressive  absorption  by  the  solid  solution  of 


ii 


in 


IV 


FIG.  46. — Changes  in  a  0.4  per  cent.  Carbon  Steel  on  Heating  and  Quenching. 

the  remaining  free  ferrite.  Quenching,  column  IV,  will  therefore 
produce  a  "  harder  "  steel  than  in  case  III,  but  nevertheless  without 
complete  refinement  of  the  steel  as  a  whole. 

Heating  to  a  temperature  D,  slightly  over  the  upper  critical 
range,  if  prolonged  for  a  length  of  time  sufficient  to  effect  complete 
diffusion  and  equalization,  will  entirely  refine  the  steel,  giving  it  the 
smallest  grain  size  possible.  Quenching,  column  V,  will  retain  this 
condition  and  give  the  maximum  hardness  possible. 


HARDENING  67 

Heating  to  a  temperature  E,  considerably  over  that  of  the  upper 
critical  range,  will  tend  to  increase  the  grain  size;  and  quenching, 
column  VI,  will  retain  this  condition,  giving  a  more  brittle  steel. 

Relation  of  Hardening  to  Annealing. — Thus  it  will  be  seen  that 
during  the  heating  operation  the  changes  taking  place  in  the  micro- 
scopic constituents  and  the  structure  as  a  whole  are  similar  in  both 
hardening  and  annealing.  The  main  difference  in  the  final  results  of 
the  two  processes  is  due  to  the  rate  of  cooling  through  the  critical 
ranges,  and,  therefore,  upon  the  nature  of  the  micro-constituents 
which  are  thereby  retained  in  the  steel  when  cold. 

The  effect  of  slow  cooling  through  the  critical  ranges,  which  is 
characteristic  of  true  annealing,  has  been  discussed;  in  brief  it  may 
be  said  that  the  austenite  or  solid  solution  shifts  its  carbon  content 
through  generating  pro-eutectoid  ferrite  (or  cementite)  to  the  eutec- 
toid  ratio  of  about  0.85  to  0.9  per  cent,  carbon,  and  then  transforms 
with  increase  of  volume  at  Arl  into  pearlite,  with  which  the  ejected 
ferrite  (or  cementite)  remains  mixed.  This  change  or  decomposition 
of  the  austenite,  however,  does  not  take  place  suddenly  or  spas- 
modically, but  develops  by  stages;  and  that  these  intermediary 
stages  between  austenite  and  its  final  constituents  may  be  recognized 
and  identified  under  the  microscope  as  martensite,  troostite,  osmond- 
ite  and  sorbite  is  generally  accepted.  Hardening  is  but  the  result 
of  obstructing  this  transition,  thereby  retaining  in  the  steel  the 
"  hard  "  austenite  or  its  early  decomposition  products  martensite 
or  troostite. 

Austenite. — Austenite  is  only  obtained  with  difficulty  in  the 
ordinary  carbon  steels,  and  even  then  is  usually  decomposed  in  part 
into  martensite.  The  two  agents  1 — rapid  cooling  and  carbon — 
tending  to  obstruct  this  transition  must  be  grouped  in  suitable  pro- 
portions— that  is,  the  carbon  content  must  be  high,  and  the  cooling 
take  place  with  extreme  rapidity.  With  about  1.5  per  cent,  carbon 
steel,  such  as  is  generally  used  in  corrugating  and  roll-turning  tools, 
when  quenched  in  brine  or  very  cold  water  from  about  1400°  F., 
about  one-half  of  the  austenite  will  remain  unaltered.  When  the 
carbon  is  about  1.1  per  cent. — which  may  be  regarded  as  about 
the  minimum  limit,  although  the  author  has  succeeded  in  obtaining 
some  austenite  with  water-quenched  0.9  per  cent,  carbon  steel — 
the  cooling  must  be  done  in  iced  solutions  from  a  temperature  of 
1800°  F.  or  more. 

1  Alloys  are  also  obstructing  agents  in  the  sense  that,  if  present  in  the  proper 
amount,  they  lower  the  temperature  at  which  the  transition  will  commence. 


68  STEEL  AND  ITS  HEAT  TREATMENT 

The  hardness  of  austenite,  as  preserved  in  hardened  high-carbon 
steels,  does  not  fall  very  far  short  of  that  of  the  accompanying  mar- 
tensite,  probably  because  the  austenite  is  partly  transformed  into 
martensite  in  cooling.  On  the  whole,  however,  austenite  may  be 
regarded  as  being  considerably  softer  than  martensite,  and  also  much 
tougher;  the  austenite  as  obtained  in  high  manganese  and  high  nickel 
steels  is  but  moderately  hard. 

Martensite. — Martensite  is  the  chief  characteristic  constituent 
of  hardened  carbon  steels  when  cooled  rapidly  in  water  from  a  tem- 
perature above  the  A3  range.  In  very  high-carbon  steels,  rapidly 


FIG.  47. — Martensite.     X75.     (Ordnance  Dept.) 

cooled,  the  martensite  is  associated  with  austenite.  In  the  lower 
carbon  steels  hardened  in  water,  in  high-carbon  steels  hardened  in 
oil,  or  in  thick  pieces  of  high-carbon  steel  hardened  in  water,  mar- 
tensite is  associated  with  troostite  and  with  some  pro-eutectoid 
ferrite  or  cementite. 

Of  the  transition  constituents — austenite  to  pearlite — martensite 
is  the  hardest  and  also  the  most  brittle,  having  extremely  high  tensile 
strength  with  little  or  no  ductility.  Microscopically  martensite  is 
characterized  by  a  needle-like  structure  as  is  shown  in  Fig.  47. 

Troostite. — Troostite  is  obtained  by  cooling  through  the  trans- 
formation range  at  an  intermediate  rate,  as  in  small  pieces  of  steel 


HARDENING  69 

when  quenched  in  oil,  or  quenched  in  water  from  the  middle  of  the 
transformation  range,  or  in  the  center  of  larger  pieces  quenched  in 
water  from  above  the  critical  range.  The  early  appearance  of  troos- 
tite  in  tool  steel  is  shown  in  Fig.  48. 

The  hardness  of  troostite  is  intermediate  between  that  of  the 
martensitic  and  pearlitic  state  corresponding  to  the  carbon  content 
of  the  specimen.  In  general,  the  hardness  increases,  the  elastic  limit 
rises,  and  the  ductility  decreases,  as  the  carbon  content  increases. 


» H '"  j  -•'>  '. 

v4vr  >< 


*^  * 


NW   > 

••••••H 


v-v   N    7'.^ 


FIG.  48.— Troostite  (Dark)  in  Hardened  Carbon  Tool  Steel.     X100.    (Bullens.) 

Sorbite. — Sorbite,  when  obtained  by  hardening,  is  ill  defined  and 
almost  amorphous;  it  is  'softer  than  troostite  for  a  given  carbon 
content.  Dependent  upon  the  carbon  content,  sorbite  may  be 
obtained  by  quenching  small  pieces  of  steel  in  oil  or  in  molten 
lead,  or  even  by  air-cooling  them;  or  it  may  be  obtained  by  quench- 
ing in  water  from  just  above  the  bottom  of  the  Arl  range.  Sorbite, 
and  to  some  extent,  troostite,  are  more  characteristic  of  tempered 


70 


STEEL  AND   ITS   HEAT   TREATMENT 


steels  than  of  hardened  steels.     The  transformation  of  troostite  into 
troosto-sorbite  is  shown  in  Fig.  49. 

Temperature  for  Hardening. — As  a  general  rule,  hardening  is 
carried  out  from  a  temperature  of  about  50°  F.  above  the  line  A3-A2.3 
-Al.2.3  in  Fig.  13.  This  is  done  to  obtain  the  best  refinement 
of  the  steel  as  well  as  the  maximum  hardening  effect.  Rapid  cooling 
of  medium  and  low-carbon  steels  from  a  temperature  just  above  the 


FIG.  49,— -Trooste-Sorbite  (Dark),     X100.     (Bullens.) 

bottom  of  the  critical  range  Al,  will  not  bring  out  the  maximum 
hardening  effect.  The  general  temperatures  most  applicable  for 
individual  steels  are  given  in  subsequent  chapters. 

When  the  maximum  results,  both  as  affecting  the  structure  and 
also  the  physical  results,  are  to  be  obtained,  experiments  should  be 
made  to  determine  exactly  how  far  over  the  critical  range  the  steel 
should  be  taken.  In  some  steels  it  will  be  found  that  approximately 
50°  F.  will  accomplish  this  purpose;  in  others  it  may  be  necessary 


HARDENING  71 

to  raise  the  temperature  to  even  150°  F.  or  more.  This  effect  will 
also  be  influenced  by  the  size  of  the  piece  to  be  hardened,  as  will 
be  shown  under  "  Tool  Steel." 

Heating  for  Hardening. — The  general  rules  for  heating  for 
hardening  may  be  simply  stated,  but  their  fullest  comprehension  and 
application  may  be  obtained  only  in  the  light  of  experience.  This 
heating  requires  much  more  care  than  heating  for  annealing  (if  such 
be  possible) ,  on  account  of  the  diametrically  opposite  functions  which 
are  indicative  of  the  two  operations.  Heating  for  annealing  is 
followed  by  slow  cooling  and  the  gradual  release  of  all  stresses 
and  strains:  heating  for  hardening  is  followed  by  the  most  severe 
test  to  which  steel  can  be  put — very  rapid  cooling,  accompanied 
by  the  setting  up  of  a  condition  of  stress  and  strain.  In  general, 
we  may  say  that  the  heating  for  hardening  should  be  slow,  uniform, 
and  thorough,  and  to  the  lowest  temperature  which  will  give  the 
desired  results. 

Non-uniformity  in  heating  must  of  necessity  result  in  lack  of 
uniformity  in  cooling,  which  in  turn  is  the  genesis  of  most  of  the 
troubles  in  the  hardening  process.  Hardening  cracks  are  more 
often  the  result  of  uneven  heating  than  of  a  defect  in  the  steel. 
Heating  requires  time  and  care.  The  peculiarities  of  each  steel  and 
article  must  be  thoughtfully  studied;  experiments  must  be  made; 
and  the  clear  judgment  of  experience  applied  to  each  individual 
case.  It  has  been  well  said  that  "  Steel  is  mercurial  and  delicately 
responsive  to  heat;  its  records  appear  in  its  own  structure." 

Lowest  Quenching  Temperature  the  Best. — The  lowest  heat 
which  will  give  the  results  desired  should  always  be  used  in  hardening. 
This  point  can  be  brought  home  in  no  better  way  than  to  give  the 
results  of  two  tests  made  which  illustrate  exactly  this  principle. 
Two  automobile  gears  were  made  from  the  same  bar,  by  the  same 
man,  and  in  all  other  ways  as  nearly  alike  as  possible.  The  tests 
were  made  by  a  disinterested  third  party. 

Number  1  was  quenched  in  oil  from  1450°  F.,  annealed  at  1400°, 
hardened  in  oil  from  1450°,  and  tempered  at  475°  in  oil.  It  gave  a 
sclerescope  hardness  of  76  to  78.  It  withstood  48  blows  of  a  10-lb. 
hammer  dropping  30  ins.  before  a  tooth  could  be  broken  out,  or  8 
blows  of  a  10-lb.  hammer  dropping  48  ins. 

Number  2  was  quenched  in  oil  from  1400°,  which  was  just 
over  the  critical  range,  and  determined  by  when  the  magnet  "  let 
go."  It  was  annealed  at  just  under  that  temperature,  followed 
by  hardening  in  oil  from  1400°  and  tempering  in  oil  at  475°.  The 


72  STEEL  AND   ITS  HEAT  TREATMENT 

hardness  was  the  same  as  with  No.  1.  In  this  case,  however,  it 
required  200  blows  of  the  10-lb.  hammer  falling  30  ins.,  or  78  blows 
with  a  fall  of  48  ins. 

The  effect  of  the  increase  of  only  50°  in  the  hardening  temperature 
is  self-evident:  it  meant  a  difference  in  efficiency  in  the  ratio  of  48 
to  200. 

Temperature  of  Quenching. — It  is  not  only  the  uniformity  of 
heating  of  the  steel  object  which  is  necessary  for  uniform  and  proper 
hardening,  but  also — and  equally  important — the  uniformity  of 
temperature  of  the  piece  at  the  moment  of  quenching.  A  piece  of 
steel  may  be  properly  heated  at  the  moment  previous  to  its  with- 
drawal from  the  furnace;  but  that  same  piece  may  have  wide  dif- 
ferences of  temperatures  in  different  parts  of  the  mass  at  the  moment 
of  quenching.  Non-uniformity  of  heat  saturation  at  the  latter 
instant  must  inevitably  result  in  non-uniformity  of  hardening — 
with  the  attendant  possibility  of  warping,  cracking  and  similar  fea- 
tures. Whether  or  not  the  indirect  cause  of  such  a  condition  is 
due  to  the  shape  and  size  of  the  object  or  to  the  method  of  handling 
the  stock  between  the  furnace  and  bath  is  immaterial  as  far  as  the 
basic  principle  outlined  above  is  concerned.  These  are  but  a  mani- 
festation of  the  ever-present  "personal  element." 

Overheating. — Overheating  is  probably  one  of  the  most  com- 
mon sins  of  the  hardening  shop.  Unfortunately,  many  "practical  " 
men  still  believe  in  the  efficacy  of  high  temperatures  for  greater 
hardening  effect.  Although  this  may — to  a  very  limited  extent — 
be  true,  the  weakening  of  the  steel  by  the  increase  in  grain  size  and 
greater  hardening  strains  as  obtained  by  high  temperatures  more 
than  offset  the  questionable  production  of  greater  hardness.  Fully 
80  per  cent,  of  the  complaints  of  "  bad  steel  "  which  have  been 
brought  to  the  author's  attention  have  been  the  direct  result  of  over- 
heating. Both  theory  and  practice  support  the  old  rule  that  "  the 
lowest  heat  which  will  give  the  desired  results  is  the  best  heat." 

The  Magnet  in  Hardening. — It  will  be  recalled  that  steel  becomes 
non-magnetic  (for  all  practical  purposes)  in  passing  through  and 
beyond  those  temperatures  represented  by  the  heavy  black  line  in 
Fig.  50.  For  steels  with  about  0.35  per  cent,  carbon  and  upwards 
this  temperature  line  also  corresponds  to  the  best  refinement  of  the 
steel  in  heating.  We  therefore  have  a  very  simple  and  practical 
means  of  determining  the  proper  temperatures  for  hardening  such 
steels.  All  that  is  necessary  is  to  apply  an  ordinary  horse-shoe 
magnet,  suspended  from  a  suitable  rod,  against  the  hot  steel.  When 


HARDENING 


73 


the  correct  hardening  temperature  has  been  reached  there  will  be 
no  attraction  between  the  magnet  and  the  steel.  For  steels  with  less 
than  about  0.3  per  cent,  carbon  the  rise  in  temperature  between  the 
Ac2  and  Ac3  ranges  may  be  estimated  and  the  steel  hardened  when 
it  has  reached  the  latter  temperature.  It  will  be  found  that  the  use 


F. 

1800 
1700 
1600 
1500 
1400 
1300 
1209 


0.2 


O.i 


0.6 


0.8 


1.0 


1.2 


FIG.  60. — Carbon-iron  Diagram  Showing  Temperatures  at  which  Ordinary- 
Carbon  Steel  Loses  Its  Magnetism  on  Heating. 

of  the  magnet  will  be  of  great  value  to  those  not  having  the  proper 
pyrometer  control  over  their  heating  operations  and  who  have  to 
depend  entirely  upon  the  eye  for  gauging  such.  An  instrument 
more  convenient  for  this  purpose  than  the  ordinary  magnet  is  made 
by  magnetizing  a  small,  elongated,  diamond-shaped  piece  of  steel, 


74  STEEL  AND   ITS  HEAT  TREATMENT 

and  supported  between  two  pins  in  the  end  of  a  forked  rod,  as  is 
shown  in  Fig.  51. 

Motion  during  Hardening. — When  a  piece  of  steel  is  quenched, 
movement  should  be  given  to  the  steel,  to  the  quenching  medium, 
or  to  both.  This  is  for  two  reasons:  (1)  in  order  to  cool  the  steel  as 
rapidly  as  possible,  and  (2)  to  break  up  any  tendency  toward  the 
formation  of  a  distinct  line  between  the  hardened  and  unhardened 
parts  of  a  differentially  hardened  forging.  The  first  factor  should  be 
self-evident:  agitation  of  the  bath  or  movement  of  the  hot  steel 
will  lower  the  temperature  of  the  oil  or  water  which  is  cooling  the 
steel,  will  prevent  the  formation  of  vapor  or  steam  around  the  steel, 
and  in  other  ways  more  rapidly  cool  the  metal.  In  the  second  place, 
if  a  piece  of  hot  steel,  such  as  a  chisel  or  die-block,  were  to  be  immersed 
to  a  certain  point  and  held  there  quietly,  the  rapid  cooling  would 
harden  the  steel  up  to  the  point  of  immersion  and  no  further;  in  other 
words,  there  would  be  a  sharp  line  of  demarkation  between  the  hard- 


FIG.  51. — A  Magnet  Used  in  Hardening. 

ened  and  unhardened  parts,  and  which  in  turn  would  be  a  source 
of  great  weakness  and  possible  fracture.  So,  in  quenching,  move 
the  piece  up  and  down  in  the  bath;  or  if  it  is  to  be  only  partly 
immersed,  agitate  the  bath  so  that  there  will  be  no  distinct  line  of 
hardening:  avoid  straight-line  hardening. 

Furnace  Equipment. — The  general  principles  of  heat  application 
will  be  discussed  elsewhere.  Some  of  the  main  points  to  be  con- 
sidered are  uniformity  of  heated  product,  quality,  and  economic 
efficiency,  and  to  conduct  all  operations  with  these  in  view.  The 
material  to  be  heated  should  be  so  arranged  in  the  furnace  that  there 
is  ample  room  for  the  circulation  of  the  heat  through  the  mass.  The 
furnace  should  be  so  designed  as  to  suit  the  class  of  work  to  be  heated, 
and  so  operated  that  the  heat  shall  be  evenly  distributed  and  of  the 
same  temperature  from  floor  to  roof  and  side  to  side.  Only  under 
special  circumstances,  such  as  in  connection  with  automatic  furnaces, 
is  it  desirable  to  operate  the  furnace  at  a  temperature  higher  than  the 
maximum  temperature  desired  in  the  steel. 

The  Human  Element  arid  Basic  Heat  Treatment  Conditions. — 
What  is  it  or  who  is  it  that  determines  when  the  charge  in  the  cham- 


HARDENING  75 

her  is  saturated  with  heat  to  the  temperature  indicated  by  the 
pyrometer? 

What  determines  the  manner  in  which  the  charge  is  placed  in  the 
furnace  and  the  room  for  circulation  throughout  the  mass?' 

What  determines  when  the  bottom  and  center  of  the  mass  are 
at  the  same  temperature  as  the  top  and  outside? 

What  regulates  the  flow  and  composition  of  gases  in  the  chamber 
around  the  stock  and  the  discharge  of  heat  from  the  chamber? 

What  determines  if  each  piece  is  heated  like  every  other  piece 
and  is  uniform  throughout,  and  whether  each  piece  goes  into  the 
quenching  bath  at  the  same  temperature  as  all  the  others  and  at  the 
temperature  indicated  by  the  pyrometer? 

What  is  it  that  controls  the  flow  of  air  into  the  furnace  or  the  flow 
of  gases  from  the  furnace,  and  by  so  doing  determines  whether  the 
atmosphere  surrounding  the  stock  is  oxidizing  or  neutral,  and 
whether  the  fuel  is  conserved  or  wasted? 

These  are  some  of  the  elements  that  affect  proper  heat  treat- 
ment, and  they  are  determined,  not  by  a  pryometer  nor  a  furnace  nor 
by  similar  apparatus  nor  by  any  mechanical  means,  but  by  the  same 
methods  that  govern  the  quality  of  the  products  of  the  kitchen — 
the  judgment  and  skill  of  the  operator. 

Heating  Baths. — Despite  the  efficiency  of  design  of  many  furnaces, 
— and  not  mentioning  those  of  poor  design — their  often  inefficient 
operation  has  a  general  tendency  towards  non-uniformity  in  heating 
and  oxidation.  In  the  effort  to  solve  these  two  problems  at  once — 
that  is,  to  surround  the  object  to  be  heated  with  a  constant  and  uni- 
form heat  on  all  sides,  and  to  avoid  contact  with  the  air — the  appli- 
cation of  various  molten  baths  has  come  about.  Chief  of  these  heat- 
ing mediums  are  molten  lead  and  certain  salts.  On  account  of  the 
operating  cost  and  necessarily  small  capacity,  however,  their  use 
is  largely  confined  to  the  heating  of  tools  and  other  articles  requir- 
ing particular  care,  uniformity,  and  freedom  from  oxidation.  The 
principaKuse  for  such  baths,  in  the  author's  opinion,  should  be  for 
the  retention  of  a  bright  surface  on  the  metal  after  hardening, 
and  not  for  uniformity  in  heating;  any  furnace,  properly  designed 
for  the  work  in  hand,  heated  with  the  right  fuel,  and  correctly  oper- 
ated, should  give  entire  uniformity  of  heating. 

Heating  in  Lead. — Before  the  advent  of  the  modern  heat  treat- 
ment furnace,  heating  in  molten  lead  represented  the  most  practical 
method  of  obtaining  uniform  heating.  With  a  reasonable  amount 
of  care  and  attention  the  typical  lead  bath  may  be  maintained  at 


76  STEEL  AND   ITS  HEAT  TREATMENT 

such  temperatures  as  the  ordinary  hardening  operation  requires  and 
with  a  satisfactory  degree  of  uniformity.  Its  use,  however,  presents 
many  difficulties.  The  bath  must  be  frequently  agitated  to  preserve 
a  uniform  temperature.  When  heated  to  over  1200°  F.  lead  begins 
to  volatilize,  giving  off  fumes  which  are  both  offensive  and  poisonous ; 
suitable  ventilation,  such  as  may  be  obtained  with  a  properly  de- 
signed hood,  should  be  provided  to  remove  these  fumes.  Further, 
the  bath  must  be  covered  with  powdered  charcoal  to  reduce  the  oxides 
or  dross  which  are  formed  in  the  molten  lead.  Many  plants  will  not 
use  lead  baths  if  temperatures  greater  than  1475°  or  1500°  F.  are 
necessary.  On  account  of  its  high  specific  gravity,  heating  in  lead 
requires  some  method  of  holding  the  steel  beneath  the  surface,  as 
otherwise  the  tool  would  float  on  the  surface  of  the  bath  and  thus  be 
unevenly  heated.  One  of  the  most  troublesome  difficulties  with 
lead  baths  is  the  tendency  of  the  lead  to  stick  in  the  holes,  threads, 
or  even  to  the  surface  of  the  tool  when  it  is  removed  for  quenching, 
so  that  uniformity  of  cooling  is  sometimes  materially  affected.  Al- 
though this  particular  difficulty  has  been  largely  eliminated  by  the 
use  of  a  paste,  the  trouble  may  simply  be  aggravated  in  case  this 
coating  has  not  been  carefully  and  properly  applied. 

Salt  Baths. — Many  of  the  difficulties  encountered  in  the  use  of 
lead  for  heating  may  be  overcome  by  the  substitution  of  different 
salts.  Their  lower  specific  gravity  permits  of  a  more  uniform 
circulation  and  there  is  no  tendency  of  the  tool  to  float  on  the  sur- 
face of  the  bath.  At  the  usual  temperatures  used  for  hardening 
there  is  little  or  no  vaporization.  Although  lead  may  prevent  the 
steel  from  oxidation  while  the  steel  is  being  heated,  as  soon  as  the 
tool  comes  in  contact  with  the  air  on  removal  from  the  lead  bath, 
a  thin  film  of  oxide  is  formed;  with  the  salt  bath,  on  the  contrary, 
the  steel  receives  a  thin  and  uniform  coating  of  molten  salt,  which 
protects  the  surface  of  the  metal. 

The  minimum  temperature  of  the  salt  bath  may  be  very  closely 
estimated  without  the  use  of  a  pyrometer.  Common  table  salt  has 
a  freezing-point  of  1472°  F.,  and  if  it  should  be  melted  with  potassium 
chloride  (freezing-point  1325°  F.)  or  other  salts,  the  freezing-point 
of  the  melt  may  be  quite  accurately  adjusted  over  a  wide  range  of 
temperatures.  By  keeping  the  bath  very  near  its  freezing-point  by 
a  suitable  regulation  of  the  heat,  overheating  of  the  steel  may  be 
entirely  overcome.  Further,  if  the  composition  of  the  salt  bath  has 
been  so  adjusted  as  to  approximate  the  proper  hardening  temper- 
ature, when  the  steel  is  removed  from  the  bath  it  may  be  quenched 


HARDENING  77 

just  at  the  time  when  the  salt  film  begins  to  solidify — or  at  exactly 
the  correct  temperature. 

BATHS   FOR   QUENCHING 

General  Properties  of  Quenching  Media. — The  main  thought  in 
selecting  a  proper  bath  for  quenching  is  the  rapidity  with  which  the 
heat  is  removed  from  the  hot  steel.  This  property  of  transference  or 
withdrawal  of  heat  from  the  solid  by  and  to  the  liquid,  will  depend 
upon  the  specific  heat  of  the  liquid,  its  conductivity,  viscosity  and 
volatility.  That  is,  the  specific  heat  will  indicate  the  heat-absorptive 
power  of  the  liquid;  the  conductivity  will  measure  its  capacity  for 
transferring  the  heat  thus  absorbed  to  the  cooler  part  of  the  bath ; 
the  viscosity  affects  the  motion  of  the  liquid  and  thus  influences  the 
uniformity  of  cooling;  and  the  volatility  indicates  the  temperature 
at  which  the  liquid  will  become  gaseous,  thus  forming  a  vapor  around 
the  steel  and  preventing  the  quick  removal  of  the  heat  from  the 
steel.  By  obtaining  a  suitable  combination  of  these  various  prop- 
erties a  bath  giving  the  desired  effect  may  be  obtained. 

Temperature  of  the  Bath. — The  continuous  use  of  any  bath  for 
quenching  will  gradually  and  progressively  raise  the  temperature  of 
the  liquid  used.  As  a  general  rule,  differences  in  the  temperature  of 
the  bath  will  give  rise  to  varying  results  in  the  actual  hardening  taking 
place — the  higher  the  temperature  of  the  bath,  the  less  its  cooling 
efficiency.  This  is  especially  noticeable  with  water;  a  change  of 
50°  or  100°  F.  will  often  entirely  alter  the  physical  properties  of  the 
quenched  steel.  The  effect  is  less  marked  with  oils,  and  with  some 
oils  may  be  almost  negligible  for  certain  classes  of  work.  On  the 
whole  it  is  decidedly  better  practice  to  maintain  as  nearly  as  possible 
a  standard  temperature  in  the  quenching  bath. 

Quenching  Speed  of  Different  Media. — It  is  evident  that  the 
cooling  medium  used,  its  temperature  and  condition  will  affect  the 
rate  of  cooling.  Matthews  1  and  Stagg  have  devoted  considerable 
time  to  investigating  numerous  commercial  media  which  are  in  use 
in  typical  hardening  plants  of  the  country  at  the  present  time. 
Their  method  was  as  follows :  A  suitable  test  piece  was  machined  from 
a  solid  bar,  and  a  hole  drilled  through  one  end  to  within  an  equal  dis- 
tance from  each  side  and  bottom  of  the  test  piece.  Into  this  hole 
a  calibrated,  platinum-rhodium  couple  was  inserted  and  the  leads 
connected  to  a  calibrated  galvanometer.  The  test  piece  was  then 

i  Matthews  and  Stagg,  "  Factors  in  Hardening  Tool  Steel,"  A.  S.  M.  E.,  1915. 


78  STEEL  AND   ITS  HEAT  TREATMENT 

immersed  in  a  lead  pot,  and  the  lead  pot  was  maintained  at  a  tem- 
perature of  1200°  F.  When  the  couple  inside  the  test  piece  was  at 
1200°  F.,  and  the  couple  in  the  lead  pot  also  read  1200°  F.,  the  test 
piece  was  removed  and  quenched  in  25  gals,  of  the  quenching  medium 
under  consideration.  At  the  start  the  quenching  medium  was  at 
room  temperature.  The  time  in  seconds  that  it  took  the  test  piece 
to  fall  from  a  temperature  of  1200°  F.  to  a  temperature  of  700°  F., 
was  noted  by  the  aid  of  a  stop-watch.  It  is  clear  that  immersing  the 
test  piece  in  the  quenching  medium  raised  the  temperature  of  the 
medium.  The  test  piece  was  then  replaced  in  the  lead,  heated  to 
1200°  F.,  quenched  into  the  medium  at  this  higher  temperature  and 
the  time  again  taken  with  the  stop-watch.  These  operations  were 
continued  until  the  quenching  medium,  in  the  case  of  oils,  had 
attained  a  temperature  of  about  250°  F.  The  results  obtained,  time 
in  seconds,  for  a  fall  from  1200°  F.  to  700°  F.,  were  plotted  against 
the  temperature  of  the  quenching  medium  and  a  series  of  curves  as 
shown  in  Fig.  52  l  were  obtained. 

The  various  curves  represent  the  following  quenching  media : 

W.  Syracuse  city  water. 

B.  Brine. 

Sec. 

1.  New  fish  oil;  average  of  readings  from  80°  to  250°  F.  .  85 

2.  No.  2  lard  oil 87 

3.  Prime  lard  oil  in  use  two  years 99 

4.  Boiled  linseed  oil 101 

5.  Raw  linseed  oil 102 

6.  New  extra-bleached  fish  oil 106 

7.  New  yellow  cottonseed  oil 107 

8.  New  tempering  oil;  60%  cottonseed,  40%  mineral.  .  .  .  122.6 

9.  New  mineral  tempering  oil 130 

10.  No.  1  dark  tempering  oil 157 . 3 

11.  Special  "  C  "  oil 164.7 

A  consideration  of  the  results  is  interesting.  Pure  water  (curve 
W)  has  a  fairly  constant  quenching  rate  up  to  a  temperature  of  100° 
F.,  where  it  begins  to  fall  off.  At  125°  the  slope  is  very  marked. 
Brine  solutions  (curve  B)  have  both  a  quicker  rate  of  cooling  and 
are  more  effective  at  higher  temperatures  than  water.  The  curve 
does  not  begin  to  fall  off  seriously  until  a  temperature  in  the 
neighborhood  of  150°  is  reached.  Where  water  at  70°  cooled  the 
test  piece  in  60  sec.,  the  brine  solutions  cooled  it  in  55  sec. 

1  For  the  sake  of  brevity  and  clearness  the  numerous  curves  as  given  by  Mat- 
thews and  Stagg  have  here  been  grouped  under  one  plot. 


HARDENING 


79 


As  is  well  known,  the  oils  are  slower  in  their  quenching  powers 
than  water  or  brine  solutions,  but  the  majority  of  them  have  a 
much  more  constant  rate  of  cooling  at  higher  temperatures  than  water 
or  brine.  The  curves  shown  in  10  and  11  are  for  thick  viscous  oils 
similar  to  cylinder  oils.  These  curves  are  particularly  interesting 
in  that  they  have  slower  quenching  abilities  at  low  temperatures  than 
at  higher  temperatures.  A  comparison  of  curves  2  and  3  shows  the 
variation  in  quenching  power  of  the  same  oil  due  to  continued  ser- 


100  150 

Time  jaSeconds 

FIG.  52, — Quenching  Power  of  Liquids. 

vice.  The  differences  in  quenching  rates  may  well  account  for 
different  results  from  the  same  steel  in  different  shops,  or  in  the  same 
shop  due  to  change  of  oil  used. 

Water  Spray  for  Hardening. — Water  sprayed  under  pressure  is 
the  quickest  agent  for  rapid  cooling  in  common  use,  exceeding  in  its 
hardening  qualities  either  brine  or  water  baths.  The  main  point  to  be 
noted  is  that  there  shall  be  sufficient  volume  and  pressure  to  prevent 
the  formation  of  a  blanket  of  steam  between  the  hot  steel  and  the 


80  STEEL  AND  ITS  HEAT  TREATMENT^ 

spray.  Its  most  common  use  is  for  such  tools  as  sledges  and  others 
requiring  a  differential  hardening,  and  for  armor  plate. 

Brine. — Brine  is  used  only  in  certain  particular  lines,  such  as  file- 
hardening,  for  which  an  extremely  hard  surface  is  required.  Unless 
the  steel  has  been  most  carefully  heated,  and  is  of  a  proper  chemical 
composition,  quenching  in  brine  is  almost  certain  to  crack  the  steel. 
This  is  particularly  true  of  large  sections,  for  in  these  the  very  sudden 
cooling  of  the  outer  surface,  while  the  center  is  still  hot,  will  set  up 
stresses  and  strains  which  will  not  be  relieved  or  equalized  in  the 
short  time  allowed,  and  with  the  inevitable  results. 

Water  Quenching. — The  author  is  a  firm  believer  in  the  use  of 
oil  for  quenching,  rather  than  water,  and  would  recommend  its  use 
whenever  conditions  permit.  Water  cools  the  steel  more  rapidly,  but 
its  more  drastic  action  increases  the  internal  strain  and  consequent 
liability  to  fracture.  For  the  low-carbon  steels,  and  for  small 
and  comparatively  simple  sections  of  the  higher  carbons,  water 
quenching  may  be  used  without  much  danger.  Of  course  in  cases 
where  it  is  required  that  the  surface  shall  be  glass-hard,  or  that 
the  maximum  tensile  strength  be  obtained,  water  quenching  is  man- 
datory. On  the  other  hand,  if  the  steel  is  to  be  given  a  full  heat 
treatment  (i.e.,  quenching  and  toughening),  the  difference  in  hard- 
ness as  obtained  by  the  two  baths  may  usually  be  nearly  equalized 
by  using  a  lower  drawing  temperature  for  the  oil-quenched  piece; 
that  is,  if  a  0.40  per  cent,  carbon  steel  forging  is  quenched  in  water 
and  toughened  at  say  1200°  F.,  approximately  the  same  static  prop- 
erties may  be  obtained  by  oil  quenching  and  a  subsequent  reheating 
to  say  1050°  F.  The  principal  objections  to  the  latter  method  are 
that  the  lower  drawing  temperature  is  not  so  easily  recognized  by  its 
color,  nor  will  the  dynamic  properties  probably  be  quite  as  high — 
though  this  last  point  is  questionable.  Generally  speaking,  however, 
oil  quenching  is  more  desirable  than  water  quenching. 

Oil  Tempering. — The  term  "  oil  tempering,"  referring  to  the 
quenching  in  oil,  is  one  which  has  become  current  in  the  trade,  so 
that  the  term,  "  hardening  "  often  refers  to  quenching  in  water  only, 
or  in  some  medium  which  will  give  an  equivalent  or  greater  hardness. 
Strictly  speaking,  the  use  of  "  tempering  "  in  this  sense  is  a  mis- 
nomer, for  it  should  be  used  as  indicative  of  a  slight  reheating  or 
"  softening  "  of  the  quenched  steel. 

Special  Quenching  Methods. — It  often  happens  that  especially 
high  tensile  results  are  desired  in  certain  large  forgings  of  such  size 
and  chemical  composition  that  direct  quenching  in  water  is  deemed 


HARDENING  81 

unwise,  and  yet  in  which  it  is  desired  to  obtain  as  near  the  maximum 
effect  of  water  cooling  as  possible.  A  method  which  has  proven  in  a 
large  measure  successful  is  to  use  a  bath  of  oil  resting  upon  an  equiva- 
lent or  greater  volume  of  cold  water.  The  forgings,  when  heated 
to  the  proper  temperature,  are  lowered  into  the  oil  for  a  few  seconds 
and  thence  into  the  water.  The  oil  forms  a  film  on  the  surface  of  the 
steel,  so  that  the  sudden  effect  of  the  water  is  somewhat  diminished 
or  retarded.  The  rapidity  of  cooling  may  be  controlled  by  the  dura- 
tion of  the  oil  quenching.  It  is  obvious  that  in  using  this  method 
there  must  be  a  sufficient  volume  of  water  under  the  oil  to  prevent 
the  formation  of  steam  and  its  consequent  danger. 

For  small  tools  or  thin  instruments  such  as  saws,  the  above 
method  may  be  so  modified  as  simply  to  have  a  film  of  oil  upon  the 
surface  of  the  water,  the  oil  in  this  case  consisting  of  some  animal 
or  vegetable  oil.  The  heated  tool  is  plunged  directly  and  evenly 
through  the  oil  film  so  that  it  enters  the  water  with  a  thin  coating 
of  burnt  oil  which  protects  it  from  the  direct  action  of  the  water  and 
lessens  the  risk  of  fracture.  The  amount  of  oil  may  of  course  be 
increased  as  desired.  The  main  objection  to  these  methods  is  the 
lack  of  uniformity  in  hardening  unless  the  operator  has  had  more  or 
less  experience. 

A  method  which  is  extensively  used  in  some  tool  works  is  that  of 
using  a  combination  of  water  and  oil  quenching,  that  is,  first  plunging 
the  tool  into  water  until  a  certain  amount  of  heat  has  been  removed, 
and  then  transfer  to  the  oil,  where  it  remains  until  cold. 

Molten  lead  is  sometimes  used  as  a  quenching  medium  for  small 
sections  in  which  great  toughness  and  only  a  moderate  degree  of  hard- 
ness is  desired.  Although  dependent  upon  the  carbon  content,  steel 
subjected  to  this  process  will  generally  be  sorbitic.  Such  treatment 
will  require  no  further  reheating. 

Other  Aqueous  Quenching  Media. — Hardeners,  at  one  time  or 
another,  have  tried  about  everything  under  the  sun  in  the  attempt 
to  discover  some  new  and  wonderful  quenching  medium  which  would 
accomplish  the  phenomenal.  The  results,  for  the  most  part,  do  not 
warrant  the  addition  of  expensive  chemicals;  and  if  the  experi- 
menters do  claim  the  marvelous,  the  "  gold-brick  "  scheme  is  gen- 
erally revealed  by  thorough  investigation. 

Some  substances,  such  as  lime,  soap,  etc.,  may  be  added  to  form 
a  protective  coating  around  the  steel.  Calcium  chloride  will  raise 
the  boiling-point  of  water  to  a  considerable  degree,  so  that  the  solu- 
tion may  be  used  at  a  temperature  up  to  150°  or  175°  F.  without 


82  STEEL  AND   ITS  HEAT  TREATMENT 

danger,  and  at  the  same  time  give  many  of  the  advantages  which 
oil  hardening  possesses.  Some  salts  increase  the  hardening  effect  of 
water;  others  purify  the  water  or  soften  it.  One  of  the  most  inter- 
esting (and  wonderful?)  combinations  which  has  come  to  the  author's 
attention  contained — by  addition— ammonia,  glycerine,  sal-ammo- 
niac, spirits  of  nitre,  ammonium  sulphate,  alum  and  zinc  sulphate! 

Differential  Hardening. — In  certain  tools,  such  as  anvil  faces, 
die  blocks,  edge  tools,  and  the  like,  it  is  desired  to  obtain  a  very  hard 
outer  part,  surface  or  edge,  to  be  "  backed  "  by  a  less  hard  and 
tougher  steel.  That  is,  the  steel  is  gradually  and  progressively  to 
change  from  extreme  hardness  to  the  opposite,  or  what  we  may 
term  differential  hardening.  This  phase  may  be  obtained  either 
by  heating  the  whole  mass  of  the  steel — as  in  die  blocks,  or  by  heating 
only  part  of  the  article — as  in  chisels;  in  either  case  that  part  which 
is  to  have  the  greatest  hardness  is  immersed  or  quenched.  By  this 
method  the  heat  is  gradually  withdrawn  from  the  part  not  immersed 
through  that  part  which  is  being  subjected  to  the  cooling  bath,  so 
that  the  mass  of  steel  as  a  whole  will  become  progressively  softer 
or  tougher  from  the  hardened  face  or  edge  to  the  opposite  side. 
Precautions  must  be  taken  to  avoid  straight-line  hardening, 

Cooling  the  Water  Bath. — Where  water  is  used  as  the  quenching 
medium  it  is  customary  to  maintain  a  flow  of  fresh,  cold  water  into 
the  quenching  tank  so  as  to  keep  a  uniform  temperature  and  purity. 
Water  which  has  been  used  for  any  length  of  time  without  renewal 
goes  "  stale  "  with  a  corresponding  loss  in  cooling  efficiency.  If  the 
cost  of  water  is  such  that  it  is  inadvisable  to  dispose  of  the  overflow 
from  the  tank,  the  hot  water  may  be  cooled  by  spraying,  cooling 
towers,  etc.,  aerated,  and  then  returned  to  the  tank. 

Cooling  the  Oil  Bath. — The  common  methods  for  cooling  the  oil- 
quenching  bath  may  be  broadly  classified  as  follows:  (1)  The  cir- 
culation of  cold  water  around,  or  through  coils  in  the  bath;  (2)  the 
circulation  of  the  oil  itself;  (3)  by  the  use  of  compressed  air. 

One  of  the  simplest  methods  for  cooling  the  oil  when  in  small 
tanks  and  not  too  constantly  used,  is  to  place  the  oil  tank  within  a 
larger  tank,  with  a  space  of  say  2  to  6  ins.  between  the  two  tanks. 
This  space  is  kept  filled  with  cold  water.  As  in  all  these  systems, 
the  intake  should  be  at  the  bottom  of  the  tank,  with  the  outlet  or 
overflow  at  the  top.  The  main  objection  to  this  method  is  the  fact 
that  the  heat  in  the  oil  must  penetrate  through  the  walls  of  the  tank 
before  it  can  be  conducted  away  by  the  water. 

The  next  type  of  cooling  makes  use  of  coils  or  radiators  placed 


HARDENING 


83 


within  the  oil  tank  and  the  circulation  of  cold  water  through  these 
pipes.  These  water  lines  are  placed  close  against  the  side  of  the 
tank  so  that  they  may  not  interfere  with  the  work  being  treated. 
From  his  own  experience,  the  author  does  not  feel  that  the  radiator 


J                                                                             L 

a!    '                                                               rj- 

II 

r 
a                                    i 

IV 

II 

. 

01   I                                                 n- 

J-n 

ft- 

FIG.  53. — Radiator  Type  of  Cooling  System. 

type  as  shown  in  Fig.  53  gives  as  great  efficiency  as  the  simple  coil 
system  of  Fig.  54.  With  the  difference  of  temperature  of  the  oil 
in  the  bottom  of  the  tank,  as  contrasted  with  the  hotter  oil  at  the 
top,  it  is  difficult  to  obtain  a  thorough  circulation  of  the  cold  water 


VJ 

1    N 

1  1 

1                               D    ) 

^1 

„                L^ 

(  C                 II 

1                         r 

V    1 

r\ 

1    1 

i                  p  ) 

/I                       -  . 

'              -  [y 

1  S              i  1 

i                 r 

M 

r\ 

A  u  

'  ?J 

(t-        n- 

\                  j 

L 

\ 

\j  —    .... 

_  L 

s 

FIG.  54. — Coil  Type  of  Cooling  System. 

through  all  sections  of  the  radiator.  Further,  this  same  difference  in 
temperature  has  the  tendency  towards  unequal  expansion  of  the  top 
and  bottom  pipes,  which  may  cause  a  leakage  of  water  into  the  oil 
and  its  attendant  dangers.  In  the  coil  system  there  is  of  necessity 
a  complete  circulation,  together  with  the  elimination  of  expansion 


84  STEEL  AND   ITS  HEAT  TREATMENT 

dangers.  These  pipes  vary  in  size  from  about  1 J  ins.  to  3  ins.  diam- 
eter; the  latter  size  has  given  excellent  satisfaction  in  a  tank 
approximately  8  ft.  wide  by  16  ft.  long  and  with  a  working 
capacity  of  about  8000  gals,  of  oil.  Guide  strips  should  be  placed 
at  intervals  along  the  coils — from  top  to  bottom — to  prevent  any 
articles  from  catching  against  the  pipes  while  the  quenched  material 
is  being  raised  out  of  the  tank. 

Circulation  and  Cooling  of  the  Oil  Itself. — The  best  results  for 
keeping  down  the  temperature  of  the  oil  bath  are  undoubtedly  to  be 
had  when  the  oil  itself  is  circulated.  The  circulation  is  continually 
bringing  cold  oil  into  the  vicinity  of  the  hot  metal,  removing  the  hot 
oil  from  the  tank,  as  well  as  giving  a  more  uniform  temperature  to 
the  bath  as  a  whole.  In  the  previous  systems  the  heat  must  be 
taken  away  by  gradual  and  progressive  transference  from  the  region 
of  the  hot  steel  towards  the  sides  of  the  tank,  and  at  the  best  is  a 
slow  procedure—this  is  assuming  that  the  oil  is  not  kept  in  motion 
by  compressed  air.  In  the  present  system,  the  heat  is  taken  away 
from  the  quenching  bath  by  the  actual  removal  of  the  hot  oil  itself. 

The  usual  methods  are  to  pump  the  hot  oil  from  the  tank  and  then 
through  coils  which  are  cooled  by  suitable  means;  or  by  maintaining 
large  supply  tanks  in  which  the  oil  will  have  sufficient  time  to  cool 
before  being  returned  to  the  quenching  tank.  In  the  former  pro- 
cedure the  coils  containing  the  hot  oil  may  be  cooled  by  refrigerating 
— such  as  the  ammonia  process,  etc. — or  by  placing  the  coils  in  a 
water  tank,  or  by  cooling  the  coils  with  a  continual  stream  or  spray  of 
water.  Where  the  size  of  plant  will  permit  the  installation  of  a 
refrigerating  system,  such  a  method  is  by  far  the  most  satisfactory; 
the  heat  may  be  removed  very  quickly,  and  the  temperature  of  the 
oil  controlled  at  any  desired  temperature  by  the  regulation  of  its 
flow  through  the  cooling  coils. 

As  an  example  of  the  water-bath  method,  one  steel  company 
pumps  the  oil  from  the  quenching  tank — holding  some  12,000  gals. — 
through  3-in.  pipes  and  thence  through  coils  placed  in  a  large  water 
tank  used  for  the  mill  supply.  The  cold  oil  then  returns  to  the 
quenching  tank  by  gravity. 

For  smaller  plants  the  coils  may  be  most  conveniently  cooled 
by  the  use  of  tiny  streams  of  water  trickling  over  the  coils.  On  the 
whole,  this  is  probably  the  most  satisfactory  system  of  all  for  small 
plants.  In  one  case  (in  which  the  question  of  the  cost  of  water 
was  important)  this  method  was  found  to  be  both  cheaper  and  to  give 
a  higher  cooling  efficiency  than  could  be  obtained  by  setting  the  coils 


HARDENING  85 

in  a  small  water  tank.  In  the  latter  case  the  heat  is  removed  by 
transference  from  one  part  of  the  water  to  that  further  removed 
from  the  coils,  so  that  unless  a  very  good  flow  is  maintained,  the 
cooling  will  be  comparatively  slow.  Further,  the  water  removed 
from  the  tank  is,  on  the  whole,  but  lukewarm,  and  therefore  but 
imperfectly  accomplishes  its  mission.  On  the  other  hand,  in  the 
drip  system  a  small  amount  of  cold  water  is  always  in  contact  with 
the  coil,  giving  a  maximum  cooling  efficiency  with  a  minimum 
expense. 

A  recent  heat  treatment  installation  l  attacks  the  problem  of 
keeping  the  quenching  medium  at  a  uniform  and  low  temperature  by 
the  maintenance  of  a  large  and  separate  supply  of  oil.  The  hardening 
is  done  in  special  quenching  tank  cars,  as  shown  in  Fig.  55,  and 
which  are  wheeled  to  any  furnace  desired.  Just  before  quenching 
commences  the  valve  in  pipe  K  is  turned  on  and  a  2-in.  stream  of  cold 
oil  is  kept  flowing  into  the  tank.  The  hot  oil  passes  out  through  the 
overflow  pipe  L,  through  the  hole  in  the  floor  and  into  a  pipe  that 
conducts  it  into  an  underground  tank.  This  underground  pipe  is 
made  very  large,  so  that  there  will  be  no  danger  of  its  clogging, 
which  would  necessitate  tearing  up  the  floor.  Each  furnace  through- 
out the  400-ft.  length  of  the  shop  is  provided  with  a  similar  inlet 
pipe  and  floor  hole  connection  to  the  pipe  which  carries  away  the 
overflow.  From  the  underground  tank  the  oil  is  pumped  to 
upright  tanks  close  to  the  outside  of  the  building;  from  these 
tanks  the  oil  flows  by  gravity  to  the  tank  cars. 

Use  of  Compressed  Air. — The  advisability  of  using  compressed 
air  in  the  -quenching  tank  is  a  much  debated  point.  If  applied 
intelligently,  however,  it  undoubtedly  renders  great  assistance  in 
the  hardening  and  cooling  operations.  In  systems  in  which  the  oil 
is  kept  in  constant  and  fairly  rapid  circulation,  it  is  neither  required 
nor  advised.2  But  if  the  oil  is  cooled  by  the  circulation  of  water 
in  pipes,  the  use  of  compressed  air  is  often  mandatory  in  order  to 
obtain  the  maximum,  as  well  as  uniform,  cooling  efficiency  of  both 
oil  and  water.  In  any  case,  the  air  must  not  be  allowed  to  come  in 
contact  with  the  hot  steel,  as  soft  spots  would  result;  neither  should  it 
be  used  in  too  great  quantities  nor  pressure,  especially  with  the 
heavier  and  low-grade  oils,  as  it  may  cause  the  precipitation  of 

1 "  A  Modern  Heat-Treatment  Plant,'7  Machinery,  Sept.,  1914. 

2  The  cold  oil  forced  into  the  quenching  tank  may  be  distributed  under  pres- 
sure to  different  parts  of  the  tank,  thus  providing  excellent  circulation,  and 
accomplishing  the  same  results  as  compressed  air. 


86 


STEEL  AND  ITS  HEAT  TREATMENT 


certain  constituents  of  the  oil,  or  cause  the  formation  of  a  scum  or 
foam  on  the  surface  of  the  oil.     When  the  air  sets  up  a  fairly  efficient 


circulation  of  the  oil  (or  water,  if  water  is  the  quenching  bath),  it 
has  accomplished  its  mission.  Compressed  air  should  rarely  be 
used  with  animal  or  vegetable  oils  on  account  of  oxidation. 


HARDENING  87 

Size  of  Quenching  Tank. — The  volume  of  the  quenching  medium 
to  be  used,  and  hence  the  size  of  the  tank,  depends  principally  upon 
the  size  and  number  of  the  pieces  to  be  hardened,  and  also  upon  the 
method  used  for  cooling  the  quenching  bath.  The  tank  should 
always  be  of  sufficient  size  to  take  with  ease  the  maximum  size  stock 
to  be  treated,  besides  a  generous  allowance  on  all  sides  for  a  suffi- 
cient body  of  oil  or  water,  for  rapidity  in  handling  the  material,  and 
for  circulation.  Further,  the  size  of  the  tank  should  be  proportioned 
to  the  degree  to  which  the  solution  can  be  kept  cooled  when  the  hard- 
ening department  is  operating  at  maximum  capacity;  the  more 
efficient  the  cooling  system,  the  smaller  the  size  of  tank  necessary. 
On  the  whole,  it  is  decidedly  preferable  to  have  the  tank  too  large 
than  too  small. 

CRACKING   AND    WARPING 

Influence  of  Non-uniformity  of  Section  on  Cracking. — One  of  the 
main  causes  of  steel  breaking  in  hardening  is  from  the  unequal  con- 
traction and  expansion  in  different  parts  of  the  steel.  If  it  were  pos- 
sible to  get  every  particle  of  the  steel  cold  at  the  same  moment  there 
would  be  an  end  to  danger  of  this  sort.  But  as  this  is  a  physical 
impossibility,  we  must  approach  such  a  condition  as  near  as  we  can. 
This  danger  of  cracking  is  particularly  emphasized  in  forgings  or 
tools  of  unequal  thickness.  If  the  thinner  part  should  be  first 
immersed  in  the  quenching  bath  (e.g.,  water),  it  would  become  cool 
much  sooner  than  the  heavier  sections;  that  is,  the  thin  part  would 
be  cold  or  "  fixed  "  while  the  thicker  part  of  the  article  was  still 
contracting  from  loss  of  heat.  Hence  the  thin  part  in  its  then  hard 
and  brittle  state  cannot  "  give  "  and  will  consequently  break;  or, 
if  it  does  not  break  at  the  time  of  hardening,  the  steel  is  held  in  such 
a  state  of  stress  that  it  is  ready  to  break  when  applied  to  the  work, 
or  even  when  being  tempered.  These  influences  are  the  more  marked 
with  the  greater  the  rapidity  of  cooling  and  hardening  effect  of  the 
bath,  as  well  as  with  the  increase  in  carbon  content  and  alloys. 

Influence  of  Bulk  of  Section  on  Cracking. — Further,  the  danger  of 
cracking  is  dependent  upon  the  bulk  of  the  article,  even  though  it 
be  of  uniform  section.  Its  effect  is  repeatedly  illustrated  by  large 
forgings  such  as  locomotive  axles,  crank-pins,  etc.,  of  rather  high 
carbons  quenched  in  water.  This  point  is  illustrated  by  the  case 
of  a  locomotive  crank-pin  which  had  been  hardened  in  water  and 
then  toughened.  A  thorough  examination  of  the  forging  before 
shipment  to  the  railroad  company  revealed  no  external  evidences 


88  STEEL  AND  ITS  HEAT  TREATMENT 

of  any  crack;  but  when  it  had  been  in  service  but  a  very  short  time 
it  fractured  badly.  Examination  then  showed  that  it  had  evidently 
been  in  a  state  of  stress  within  its  center,  with  the  development  of 
an  embryo  crack;  the  dynamic  stresses  to  which  it  had  been  sub- 
jected in  service  were  sufficient  to  raise  the  tension  beyond  what  the 
steel  would  stand,  with  the  resultant  internal  fracture  and  its  pro- 
gressive development  into  complete  rupture. 

Expansion  and  Contraction. — In  view  of  recent  research  work 
this  phenomenon  of  cracking  may  be  explained  in  a  theoretical 
manner  along  the  following  lines.  We  know  that  when  a  piece 
of  steel  is  heated  through  the  critical  range  the  formation  of 
austenite  takes  place  with  a  decrease  in  volume;  and  a  somewhat 
corresponding  and  opposite  increase  in  volume  occurs  when  it  is 
cooled  through  the  same  critical  range.  Now  if  a  large  forging  of 
considerable  diameter  is  quenched  rapidly,  the  outer  sections  will 
be  held  in  the  hardened  condition,  and  therefore  rigid  and  stressed. 
Meanwhile  the  interior  of  the  steel,  being  cooled  much  less  rapidly, 
will  in  all  probability  actually  pass  from  the  austenitic-martensitic 
condition  into  that  of  pearlite,  accompanied  by  the  increase  of 
volume  noted  above.  If  the  outer  portion  or  surface  of  the  steel 
is  unable  to  withstand  this  expansive  force,  rupture  must  necessarity 
occur.  Illustrative  of  this,  the  author  has  seen  heavy  locomotive 
axle  forgings,  after  removal  from  the  oil-hardening  bath,  actually 
break  open  with  a  tremendous  report.  However,  if  the  forging 
has  not  been  hardened  too  drastically,  and  is  removed  from  the 
quenching  bath  before  entirely  cold,  an  immediate  reheating  or 
toughening  process  will  generally  relieve  these  stresses  before  any 
actual  damage  takes  place. 

Hollow  Boring. — In  order  to  avoid  such  dangers,  there  appears  to 
be  a  decided  tendency  toward  requiring  the  drilling  of  axles,  shafts 
and  heavy  forgings  of  large  diameters  to  provide  for  heat  treatment 
and  to  remove  defective  material.  It  is  undoubtedly  the  fact  that 
heat  treatment  will  not  attain  its  full  effects  in  the  core  of  a  large 
section.  With  a  solid  axle,  the  heat,  upon  quenching,  is  removed 
by  a  flowing  from  the  center  to  the  outside  and  thence  to  the  hard- 
ening bath;  the  amount  of  heat  is  so  great,  however,  that  at  the  best 
the  core  will  be  but  semi-hardened,  and  in  most  cases  will  but  be 
grain-refined,  or  annealed.  This  point  was  well  illustrated  by  one 
company  in  its  experiments:  it  split  open  a  large,  heat-treated 
driving  axle;  the  fracture  showed  that  the  heat  treatment  had 
penetrated  the  ends  to  a  depth  of  about  6  or  8  ins.,  and  on  the 


HARDENING 


89 


sides  to  a  depth  of  about  one-half  the  radius;  the  fracture  of  the 
core  was  similar  to  that  of  annealed  steel.  Again,  the  loss  of  duc- 
tility and  failure  of  the  heat-treatment  process  thoroughly  to  pene- 
trate the  core  of  semi-hardened  steel  are  shown  by  the  following 
results  obtained  from  a  12-in.  axle,  heat  treated,  and  taken  at  regular 
intervals  from  the  center  to  the  outside: 


Tensile  Strength. 
Lbs.  per  Sq.  In. 

Elastic  Limit, 
Lbs.  per  Sq.  In. 

Elongation, 
per  Cent  in  2  Ins. 

Reduction 
of  Area,  per  Cent. 

1.  (Center) 

95,000 

60,000 

7.5 

9.6 

2. 

99,750 

60,000 

15.0 

35.3 

3. 

104,500 

65,000 

17.5 

35.7 

4. 

104,500 

65,500 

19.0 

40.3 

5.  (Outside) 

106,500 

70,000 

21.5 

47.7 

Treatment:   Quenched  in  water  from  1580°  F.;  toughened  at  1100°  F. 
Analysis:    Carbon,  0.35;    manganese,   0.56;   phosphorus,  0.020;   sulphur,  0.024;   nickel, 
1.19;   chrome,  0.31. 

By  means  of  drilling  a  hole  through  the  axle,  the  quenching  solu- 
tion is  able  to  remove  the  heat  from  both  the  inner  and  outer  part 
of  the  axle  at  the  same  time.  Hollow-bored  axles  should  be  quenched 
vertically  whenever  possible,  and  a  constant  flow  of  the  oil  or  water 
through  the  bore  be  supplied. 

The  American  Railway  Master  Mechanics'  Association  in  its 
proposed  specifications  for  alloy  steel  locomotive  forgings  (June,  1914) 
calls  for  "  drilling  forgings  over  7  ins.  in  diameter,  unless  otherwise 
specified  by  the  purchaser.  The  committee  has  found  a  great  tend- 
ency among  users  of  quenched  and  tempered  steel  to  require  drilling 
of  parts  over  7  ins.,  and  this  practice  is  advocated  by  steel-makers. 
In  the  case  of  axles  and  crank-pins  particularly,  drilling  takes  away 
practically  nothing  from  the  strength  of  the  part;  it  removes  the 
material  from  the  center  where  defective  material  is  most  likely  to 
exist  and  where  it  is  least  subject  to  the  beneficial  effects  of  heat 
treatment,  and  it  allows  the  forging  to  adapt  itself  to  expansion  and 
contraction  due  to  heating  and  cooling." 

Warping. — Warping  is  but  another  manifestation  of  the  effect 
of  unequal  contraction  and  expansion,  originating  mainly  in  incorrect 
heating  or  neglect  in  the  manner  of  quenching,  rather  than  in  the 
more  drastic  effect  of  the  bath  itself.  Non-uniform  heating  must 
inevitably  result  in  warping,  for  if  some  parts  are  hotter  than  others 
when  the  steel  is  quenched,  it  is  evident  that  the  rate  of  cooling  over 
the  entire  length  of  the  piece  cannot  be  the  same.  The  general 


90  STEEL  AND  ITS  HEAT  TREATMENT 

tendency  will  be  for  bars  to  buckle  or  twist,  due  to  unequal  contrac- 
tion during  hardening.  Take,  for  example,  a  bar  which  has  been 
placed  upon  the  relatively  cold  floor  of  a  heating  furnace  in  which 
the  main  heat  application  comes  from  above.  Under  these  condi- 
tions the  tendency  will  be  for  the  bar  to  become  more  heated  along 
the  upper  surface  than  in  that  in  contact  with  the  cold  floor.  If 
the  bar  should  now  be  quenched,  the  under  part — being  lower  in 
temperature — would  contract  first  (provided  it  were  heated  and 
quenched  from  a  temperature  over  the  critical  range)  and  thus 
become  bowed.  But  if  the  temperature  in  the  cooler  part  of  the  bar 
were  under  the  critical  range,  the  tendency  would  be  to  bend  in  the 
opposite  direction.  Other  variations  in  heating  might  give  a  double 
bend;  certain  localized  heating  might  even  cause  twisting  or  tor- 
sional  strains. 

Manner  of  Quenching. — Uniformity  of  quenching  is  requisite 
to  good  hardening  work.  As  a  general  rule,  objects  should  be 
quenched  vertically  in  the  direction  of  their  greatest  length.  Like 
all  rules,  there  are  certain  exceptions  which  must  be  made  to  this 
general  statement — such  as  in  the  case  of  half-rounds  and  articles  of 
a  corresponding  design,  as  well  as  in  such  cases  where  economic 
handling  requires  other  methods,  as  with  shafts,  small  axles,  plates, 
etc.  But  where  no  special  facilities  have  been  designed  for  uniform 
quenching,  the  above  rule  will  be  found  worthy  of  adoption  for 
symmetrical  sections,  and  especially  with  unskilled  workmen. 

The  reasons  for  this  may  be  best  explained  by  taking  small 
automobile  drive-shafts  as  an  example.  In  pulling  the  piece  out  of 
the  furnace  with  the  tongs,  the  tendency  is  to  grasp  it  nearer  the 
end  than  at  the  middle;  consequently,  in  the  general  haste  to  get  the 
steel  into  the  quenching  bath  as  soon  as  possible,  the  average  work- 
man is  very  apt  to  drop  or  plunge  it  into  the  oil  or  water  at  an  angle — 
that  is,  one  end  of  the  piece  strikes  the  quenching  solution  before  the 
remainder  of  the  steel.  Hence,  initial  hardening  strains  are  set  up 
which  usually  result  in  a  bent  shaft  when  it  is  removed  from  the 
tank.  It  is  very  difficult,  in  the  space  of  a  second  or  two,  to  get 
hold  of  the  bar  exactly  at  the  middle  and  also  to  lower  it  into  the 
water  or  oil  so  that  both  ends  are  immersed  at  the  same  identical 
moment — which  this  method  of  quenching  demands.  Now  if  the 
workman  was  to  aim  at  immersing  the  piece  end  foremost,  as  in  Fig. 
56,  grasping  it  near  the  end  (as  usual)  with  his  tongs,  the  weight  of 
the  shaft  would  automatically  tend  to  bring  the  shaft  to  the  normal, 
and  the  quenching  would  be  more  nearly  uniform,  Axles  and 


HARDENING  91 

forgings  of  a  similar  nature  should  be  quenched  vertically  whenever 
possible,  as  less  strains  are  set  up  in  the  axle  by  this  mode  of  quench- 
ing. Extensive  investigations  by  one  locomotive  builder  would  tend 
to  show  that  axles  quenched  horizontally  (as  is  customary)  develop 
a  series  of  stresses  which,  when  plotted,  appear  as  an  oval  around  the 
axis  of  the  axle  instead  of  as  a  circle. 


FIG.  56. — Proper  Method  of  Quenching  Small  Round  Bars. 

Hollow  forgings,  such  as  guns,  hollow  tools,  etc.,  should  always 
be  quenched  vertically,  so  that  the  quenching  medium  may  have  a 
free  flow  through  the  bore,  and  also  to  prevent  the  pocketing  of  any 
steam  or  vapor  which  may  be  formed  by  the  contact  of  the  hot  steel 
and  the  solution. 

Round  Sections. — The  hardening  of  round  sections  without 
cracking  or  bending,  and  without  undue  labor  cost,  presents  a  problem 


92  STEEL  AND  ITS  HEAT  TREATMENT 

which  has  attracted  much  study.  The  danger  of  fracture,  especially 
of  internal  origin — whether  actual  or  potential — is  always  greatest 
in  the  circular  section.  This  is  largely  due  to  the  fact  that  all  the 
stresses  and  their  subsequent  strains  are  grouped  symmetrically  and 
converge  upon  the  central  axis.  Both  the  square  bar  with  its  corners, 
and  the  plate  or  sheet  with  its  larger  surface  exposure,  can  more 
easily  yield  to  the  internal  stresses  and  afford  relief — either  in  cooling 
during  hardening  or  in  the  reheating  for  tempering  or  toughening — 
than  can  the  circular  section.  Further,  there  is  greater  danger  of 
bending  and  twisting  due  to  non-uniform  cooling  in  the  long,  round 
bar  than  in  almost  any  other  common  section.  As  has  been  noted, 
short  lengths  of  rounds  of  small  diameter  should  always  be  quenched 
vertically.  But  when  it  comes  to  the  handling  of  large  numbers  of 
larger  bars,  either  of  greater  length  or  diameter,  this  method  is 
obviously  at  a  disadvantage.  Yet  if  the  bars  are  simply  dropped  into 
the  bath  by  hand,  even  if  every  effort  is  made  to  have  the  axis  of  the 
bar  parallel  to  the  surface  of  the  quenching  medium,  general  unsatis- 
factory results  are  obtained,  due  to  non-uniform  cooling. 

One  satisfactory  method  for  quenching  such  bars  is  shown  in 
Fig.  55,  in  which  automobile  shafts  are  handled.  The  bars,  after 
careful  heating,  are  pulled  out  with  long  rods  which  have  a  hooked 
end,  across  the  inclined  steel  fore-hearth  J,  whence  they  drop  on  to  a 
jointed  rack  in  the  oil  tank  and  are  quenched.  By  starting  the  bars 
with  their  axes  parallel  to  the  surface  of  the  oil,  they  must  neces- 
sarily be  held  in  the  same  relative  position  as  they  pass  down  the 
rack  into  the  oil.  The  rolling  also  effects  a  more  uniform  cooling 
of  the  shaft  in  relation  to  its  central  axis.  Fig.  57  shows  how  the 
traveling  crane  lifts  one  side  of  this  jointed  rack  to  raise  the  shafts 
out  of  the  oil  and  dumps  them  on  to  the  truck  at  the  side. 
i  v  An  improvement  on  this  method  to  give  further  uniformity  in 
cooling,  and  which  has  been  used  on  finished  shafts  with  almost  the 
entire  elimination  of  bending,  is  illustrated  in  principle  in  Fig.  58. 
The  apparatus  consists  of  a  number  of  inclined  planes  or  racks 
(similar  to  that  shown  in  Fig.  57),  made  from  small  bars  or  old  rails 
which  are  held  in  position  by  suitable  cross-pieces.  The  hot  shaft 
is  started  down  the  first  plane  and  passes  into  the  oil  or  water; 
thence  it  drops  to  the  next,  and  so  on  until  it  reaches  the  bottom, 
and  is  removed  by  suitable  methods.  Notice  that  the  change  from 
one  plane  to  the  next  causes  a  reversal  in  the  direction  of  rolling,  so 
that  any  stresses  set  up  by  one  plane  are  practically  counteracted 
by  the  next  plane,  giving  a  maximum  uniformity  in  cooling.  The 


HARDENING 


93 


angle  of  incline  has  a  great  deal  to  do  with  the  practical  working  out 
of  the  procedure,  and  should  be  varied  according  to  the  diameter  of 


the  bar,  its  chemical  composition,  and  the  nature  of  the  quenching 
medium.  The  rate  of  travel  down  the  incline  should  not  be  too 
rapid,  but  should  nevertheless  be  sufficient  to  allow  the  reversing 


94 


STEEL  AND   ITS  HEAT  TREATMENT 


action  of  the  several  planes  to  take  its  effect  before  the  steel  is  to'd 
cold.  The  angle  of  the  planes  may  be  increased  as  greater  depth  in 
the  solution  is  reached.  The  bar  should  be  cold  when  it  reaches  the 
bottom  of  the  tank.  The  angle  of  the  first  incline  is  the  most 
important,  and  should  be  determined  by  experiment;  it  will  gen- 
erally be  in  the  vicinity  of  10°  or  15°.  In  one  plant  in  which  this 
method  was  used  the  number  of  shafts  requiring  straightening  was 
reduced  from  a  very  high  percentage  to  less  than  1  per  cent,  of 
the  total  number  treated. 


i     FIG.  58. — Rough  Sketch  of  Inclined  Racks  for  Quenching  Rounds. 


Double  Quenching. — The  effect  of  a  double  quench  is,  as  a  general 
rule,  to  raise  the  elastic  limit  and  tensile  strength  without  diminishing 
the  ductility.  This  is  for  the  most  part  due  to  the  higher  degree  of 
refinement  which  this  double  quenching  makes  possible,  thus  putting 
the  steel  in  the  best  possible  condition.  If  the  steel  is  in  good  con- 
dition (i.e.,  refinement)  before  the  first  quenching,  the  influence  of 
the  second  quenching  will  be  the  less  in  proportion.  It  is  often  cus- 
tomary first  to  quench  from  a  temperature  100°  or  200°  F.  over  the 
critical  range,  and  then,  for  the  second  quenching,  to  heat  just  enough 
over  the  critical  range  to  obtain  the  degree  of  hardness  desired. 


HARDENING  95 

For  high-carbon  steels  the  double  quenching  is  not  to  be  recom- 
mended except  under  unusual  conditions — such,  for  example,  when 
the  steel  has  been  greatly  overheated  in  some  previous  operation. 
The  hardening  of  high-carbon  steels  is  at  best  a  difficult  operation, 
and  the  less  heating  to  which  such  steel  is  subjected  the  better. 

Manganese  on  Hardening. — As  we  have  previously  mentioned, 
the  presence  of  manganese  causes  a  greater  hardening  effect,  due  to 
its  obstructing  the  austenite  transition.  This  increase  in  hardness — 
in  ordinary  carbon  steels  with  less  than  1.75  per  cent,  manganese — 
is  commonly  thought  to  be  associated  with  an  increase  in  brittle- 
ness,1  and  with  the  danger  of  cracking  during  or  immediately  sub- 
sequent to .  quenching.  Forethought  must  therefore  be  used  in 
obtaining  the  proper  combination  of  manganese,  carbon,  and  rate  of 
cooling  to  avoid  the  latter  difficulty.  The  general  limits  of  safety 
for  practical  work  may  be  broadly  (but  not  invariably)  set  somewhat 
as  follows:  water  quenching  is  always  dangerous  when  the  mangan- 
ese content  runs  up  around  1.50  per  cent.,  even  in  low-carbon  steels; 
with  approximately  1.00  per  cent,  manganese  water  quenching  may 
be  used — although  not  advised — with  mild  forging  steels;  with  the 
progressive  increase  in  carbon  the  manganese  content  should  be 
rapidly  lowered,  so  that  in  tool  steels  for  water  hardening  the  mangan- 
ese is  under  0.40  per  cent.,  and  with  very  high-carbon  tools  is  not  over 
0.25  per  cent.  Dependent  upon  the  size  and  general  shape  (design) 
of  the  piece,  as  well  as  the  condition  (refinement)  of  the  steel,  oil 
quenching  is  generally  safe  up  to  1.75  per  cent,  manganese  with 
0.60  per  cent,  carbon — in  fact,  one  well-known  oil-hardening  tool 
steel  analyzes  about  0.90  per  cent,  carbon  with  1.60  per  cent,  man- 
ganese. The  subject  of  high  manganese  steels  will  be  considered 
under  a  separate  chapter. 

1  Refer  to  Chapter  XV  for  a  further  discussion  of  this  point. 


CHAPTER  V 
TEMPERING   AND   TOUGHENING 

TEMPERING 

Tempering. — When  a  piece  of  carbon  tool  steel  is  heated  to  a 
red  heat  and  quenched  in  water  (i.e.,  hardened),  the  steel  becomes 
hard,  brittle,  and  is  held  in  such  a  state  of  stress  that  its  use — 
except  in  a  few  particular  cases — would  be  highly  inadvisable. 
This  hardening  operation  has  arrested  the  austenitic  transition  at  the 
martensitic  stage,  and  prevented  it  from  advancing  further,  as  into 
troostite,  etc.  Under  these  circumstances,  the  application  of  heat 
will  now  accomplish  two  results:  (1)  it  will  relieve  the  hardening 
strains,  and  (2)  permit  the  transition  to  proceed.  By  properly 
adjusting  the  temperature  of  this  reheating  process,  any  desired 
stage  in  the  martensite-troostite  transition  may  be  obtained.  And 
by  permitting  just  the  right  amount  of  the  hard,  brittle  martensite 
to  go  over  into  the  softer  and  tougher  troostite,  any  desired  combina- 
tion of  physical  properties  within  the  capacity  of  that  particular 
steel  may  be  realized.  This  process  of  "  letting  down  "  or  softening 
is  called  tempering. 

Troostite. — If  the  steel  has  been  fully  hardened  so  that  it  consists 
entirely  of  martensite,  troostite  will  begin  to  form  at  somewhere  in 
the  vicinity  of  400°  F.,  or  possibly  lower.  As  the  tempering  tempera- 
ture is  progressively  raised,  the  troostite  increases  in  amount  until 
at  about  750°  F.  it  begins  to  change  into  sorbite.  Thus  steel  in  the 
tempered  condition  is  usually  characterized  by  the  presence  of  more 
or  less  troostite,  dependent  upon  the  degree  of  hardening  and  upon 
the  tempering.  Just  as  martensite  may  be  said  to  represent  the 
condition  of  hardened  steel,  or  pearlite  that  of  annealed  steel,  so 
troostite  is  indicative  of  a  tempered  steel — whether  it  be  obtained 
by  water  quenching  and  reheating,  or  by  quenching  in  some  less 
drastic  medium  such  as  oil  but  with  no  reheating.  The  question  of 
whether  troostite  represents  a  complete  step  in  the  transformation 
is  not  definitely  known,  and  as  far  as  practical  heat-treatment  work 

06 


TEMPERING  AND  TOUGHENING 


97 


is  concerned  is  but  a  question  of  scientific  value;  the  value  of  troostite 
in  its  influence  upon  the  hardness  and  allied  properties  of  tempered 
steel  is,  however,  definitely  recognized. 

Hardening  Strains. — It  should  be  always  remembered  that 
tempering  not  only  softens  the  steel  through  the  influence  of  troostite, 
but  also  relieves  the  strains  set  up  in  hardening.  This  last  factor 
should  not  be  lost  sight  of,  for  although  the  proper  degree  of  hard- 
ness is  requisite  for  specific  work,  no  tool  will  eventually  prove  of 
much  value  if  it  retains  the  state  of  strain  occasioned  by  rapid 
cooling.  This  statement  applies  not  only  to  water  quenching,  but 
also  to  oil  quenching  (or  oil  tempering) .  Even  the  influence  of  boil- 
ing water  is  often  sufficient  to  relieve  more  or  less  of  these  strains, 
if  it  is  not  desired  to  further  soften  the  steel  by  higher  reheating. 
Naturally,  however,  the  higher  the  softening  temperature  the  better 
will  be  the  condition  of  the  steel  in  this  regard. 

Temper  Colors. — Nature  has  provided  a  useful  and  more  or  less 
empirical  indication  of  the  degree  to  which  tempering  has  affected 
the  steel  through  the  formation  of  a  surface  film  of  oxide  colors  (oxide 
of  iron).  If  a  piece  of  hardened  steel  is  brightened  with  emery 
paper  or  other  suitable  means,  and  is  then  slowly  heated  with  expo- 
sure to  the  air,  the  brightened  surface  will  take  on  characteristic 
"  temper  colors."  These  commence  with  a  very  faint  yellow  and 
progressively  change  with  increase  of  temperature  through  varying 
degrees  of  yellow,  brown,  purple  and  blue.  That  these  colors  bear 
a  definite  relation  to,  and  are  closely  indicative  of,  a  known  tempera- 
ture, under  certain  conditions,  is  now  a  generally  accepted  fact. 
Although  a  difference  in  distinguishing  the  various  shades  of  color 
is  bound  to  occur  on  account  of  the  "  personal  equation,"  the  follow- 
ing table  is  fairly  representative : 


Temper- 

Temper- 

ature, De- 
frees 

Color. 

ature,  De- 
frees 

Color. 

ahr. 

* 

ahr. 

420 

Very  faint  yellow 

510 

Brown 

430 

Yellowish-white  or  light  straw 

520 

Brown  purple  (peacock) 

440 

Light  yellow 

530 

Light  purple 

[450 

Pale  yellow  straw 

540 

Purple 

'460 

Straw 

550 

Dark  purple 

470 

Dark  Yellow 

560 

Light  blue 

480 

Deep  straw 

570 

Blue 

490 

Yellow  brown 

600 

Dark  blue 

500 

Brown  yellow  , 

625 

Blue  tinged  with  green 

98  STEEL  AND  ITS  HEAT  TREATMENT 

Limitation  of  Color  Method. — The  previous  statement  regarding 
the  relation  of  tempering  colors  to  temperature  is  true  in  its  entirety 
only  under  certain  definite  conditions  of  heating,  and  which  are 
largely  dependent  upon  the  time  element.  So  long  as  the  heat  of 
the  steel  is  being  progressively  raised — that  is,  so  long  as  the  temper- 
ature of  the  fire,  furnace  or  tempering  plate  is  greater  than  the 
temperature  of  the  steel — the  temper  colors  indicate  the  temperature 
of  that  part  of  the  steel  most  affected — the  surface.  But  when  the 
steel  is  being  kept  at  a  definite  tempering  temperature  for  any  length 
of  time,  the  colors  do  not  represent  the  actual  temperature.  This 
point  is  readily  illustrated  by  heating  a  small  piece  of  hardened  steel 
at  a  constant  temperature  for  a  considerable  period  of  time.  Thus, 
in  one  instance,  a  straw  color  was  produced  in  about  a  minute,  but 
changed  to  a  brown  in  about  ten  minutes,  and  to  a  purple  in  about 
forty  minutes;  and  yet  the  temperature  of  the  steel  was  never 
higher  than  460°  F.,  representative  of  the  straw  color.  In  other 
words,  the  time  element  has  developed  a  new  set  of  conditions  w.hich 
may  greatly  affect  the  depth  of  oxidation  or  color. 

On  the  other  hand,  it  is  a  debatable  point  as  to  whether  or  not 
these  temper  colors  represent  the  actual  condition  (not  the  temper- 
ature) of  the  steel  itself.  Some  tool  makers  maintain  that  the 
efficiency  of  the  tool — both  in  hardness  and  in  other  properties — is 
the  same  whether  the  color  has  been  obtained  by  a  short  heating  at  a 
high  temperature,  or  a  longer  heating  at  a  lower  temperature.  That 
is,  the  ultimate  results  are  indicated  by  the  temper  color,  independent 
of  the  method  of  obtaining  it.  Others  aver  that  such  is  not  the  case. 

Tempering  for  Depth. — It  is  obvious  that  the  temper  color  is 
at  the  best  but  a  surface  indication.  For  some  tools  or  articles  which 
require  a  specific  superficial  hardness  only,  and  in  which  the  condition 
of  the  center  of  the  tool  is  of  little  consequence,  it  probably  does 
not  matter  a  great  deal  in  the  ultimate  results  whether  the  temper 
color — a  straw  color  for  example — has  been  obtained  by  a  few  min- 
utes' heating  at  460°  F.,  or  by  heating  for  a  longer  period  at  say  360° 
F.  Contrariwise,  if  the  tool  or  part  is  to  be  subjected  to  stresses  of 
such  nature  as  demand  the  best  that  the  steel  is  capable  of,  the 
greatest  degree  of  uniformity  and  release  of  hardening  strains  is 
requisite.  Such  may  only  be  obtained  by  a  thorough  heating  at  a 
specified  temperature,  and  which  may  be  entirely  independent  of  the 
color  indication.  In  such  cases,  to  use  the  above  temperatures,  the 
thorough  heating  at  360° — it  more  uniformly  affecting  the  whole 
mass  of  the  steel — might  prove  immeasurably  better  than  the 


TEMPERING  AND  TOUGHENING  99 

incidental  surface  heating  to  460°.  And  as  will  be  mentioned  later, 
a  continued  heating  at  460°  would  again  be  an  improvement  over 
either  color  method. 

Quenching  after  Tempering. — The  method  of  tempering  by  color 
indication  inherently  requires  immersion  when  the  specified  color 
is  reached  to  prevent  any  further  rise  in  temperature,  or  in  the 
blacksmith's  phrase,  to  "  set  the  grain."  Although  it  is  possible 
so  carefully  to  heat  the  steel  that  the  maximum  effect  is  just  to 
develop  the  color  desired — and  no  further,  such  methods  take  so 
much  time  and  patience  that  they  are  rarely  carried  out  in  practice. 
The  necessity  of  such  immersion  or  quenching,  even  in  the  hands 
of  an  experienced  hardener,  is  the  source  of  many  troubles.  Not  only 
does  the  quenching  probably  induce  further  strains  into  the  steel, 
but  it  is  also  entirely  inconsistent  with  uniformity  of  results.  If  the 
object  is  of  considerable  size,  or  varies  greatly  in  dimension  of 
adjoining  sections  to  be  similarly  tempered,  or  is  of  intricate  design, 
the  difficulty  in  obtaining  the  same  temper  throughout  even  on  the 
surface  (to  say  nothing  of  the  interior  of  the  steel),  will  be  greatly 
magnified.  If  the  proper  color  is  reached  on  one  part  before  another, 
there  will  be  a  corresponding  difference  in  hardness.  And  thus  the 
difficulties  multiply  ad  infinitum. 

Use  of  Liquid  Baths. — Later  methods  involving  the  use  of  liquid 
baths  for  heating  overcome  the  difficulties  in  color  tempering, 
eliminate — as  a  general  rule — the  necessity  for  quenching,  and 
further  give  complete  uniformity  of  heating  throughout  the  whole 
mass  of  the  steel  and  the  maximum  elimination  of  hardening  strains 
as  can  be  obtained  at  the  temperature  used.  By  maintaining  the 
bath  at  the  proper  temperature  there  can  be  no  overheating,  the 
heat  must  penetrate  all  parts  of  the  steel  alike,  and  the  "  personal 
equation  "  is  as  nearly  eliminated  as  is  possible.  This  method  has 
the  further  advantage  of  cutting  down  labor  costs  and  increasing 
the  output,  since  a  number  of  pieces  may  be  heated  at  the  same 
time,  and  while  one  lot  is  being  tempered  another  bath  may  be 
charged  or  discharged. 

Comparison  of  Physical  Properties  Obtained. — An  excellent 
example  of  the  efficiency  of  bath  tempering  is  illustrated  in  auto- 
mobile gears.  On  account  of  the  relatively  thin  section  of  the  teeth 
as  compared  with  the  mass  of  the  gear,  exact  tempering  by  the  ordi- 
nary temper-color  practice  is  rather  difficult.  The  teeth,  which 
should  be  the  hardest,  take  the  temper  first,  and  are  therefore  the 
softest  part  of  the  gear  as  a  whole.  If  the  gears  were  to  be  tern- 


100  STEEL  AND   ITS  HEAT  TREATMENT 

pered  by  revolving  on  a  hot  bar  much  better  results  would  be  ob- 
tained than  by  ordinary  tempering,  but  the  time  and  cost  elements 
would  prove  excessive  where  hundreds  of  pieces  were  to  be  handled. 
By  the  use  of  a  suitable  liquid  tempering  bath  thorough  uniformity 
could  be  obtained  throughout.  Where  by  the  color  method,  the  core 
of  the  gear  would  have  the  tendency  to  be  too  hard,  the  teeth  per- 
haps too  brittle  or  soft  in  places,  and  only  the  surface  of  the  gear 
as  a  whole  affected  by  the  temper-color  representing  say  475°  F., 
by  the  more  modern  method  the  whole  mass  of  the  gear  would  have 
the  physical  properties  as  characterized  the  drawing  temperature 
of  the  475°  F. 

Exact  Temperatures. — Too  much  attention  cannot  be  given  to 
the  necessity  of  obtaining  exact  temperatures  in  the  tempering  opera- 
tion. For  the  average  run  of  carbon  tools  the  tempering  range  is 
very  narrow,  probably  within  a  hundred  degrees  for  the  great 
majority.  The  tempering  action  takes  place  extremely  rapidly  and 
often  a  difference  of  15°  or  20°  may  cause  much  trouble.  Trying 
to  temper  tools  over  an  open  fire  may  be  all  right  in  isolated 
cases,  but  it  spells  failure  if  made  a  general  practice. 

Tempering  Methods. — The  procedure  to  be  employed  in  temper- 
ing must  necessarily  depend  upon  the  nature  of  the  tool  or  part. 
Methods  must  be  developed  to  satisfy  the  individual  requirements 
and  are  too  numerous  to  discuss  here.  Briefly,  however,  the  more 
common  practices  may  be  covered  by  the  tempering  plate,  the  sand 
bath,  and  such  liquid  baths  as  oil,  lead  and  alloys,  and  molten 
salts. 

Tempering  Plate. — The  tempering  plate  generally  consists  of 
an  iron  casting  planed  smooth  on  top,  and  heated  from  beneath  by 
suitable  means,  such  as  gas,  oil,  or  even  a  coal  or  coke  fire.  The 
steel  articles  are  placed  on  the  plate  and  moved  about  until  they 
have  attained  the  proper  temper  color  and  then  quenched.  Fig.  59 
shows  a  characteristic  equipment  for  heating,  hardening  and  temper- 
ing dies;  Q  represents  the  discharge  end  of  the  heating  furnace, 
R  the  quenching  tank,  and  T  the  tempering  plate,  the  latter  being- 
heated  by  oil  burners  from  beneath. 

Sand  Bath. — In  order  to  effect  more  uniform  tempering  of  small 
tools,  a  pan  of  clean,  well-dried  sand  may  be  placed  on  a  suitable 
hot-plate,  or  in  a  furnace.  The  sand  is  held  at  the  desired  tem- 
perature, which  may  be  determined  by  the  insertion  of  a  ther- 
mometer or  pyrometer  couple,  and  may  be  protected  by  covering 
with  a  suitable  hood.  The  oxide  colors  on  the  steel  may  also  be 


TEMPERING  AND   TOUGHENING 


101 


used  as  a  measure   of  the  tempering,  as   there  is  of  course  free 
access  of  air  between  the  particles  of  the  sand. 

Oil  Baths. — For  much  of  the  ordinary  tempering  work  an  oil 
bath  will  probably  prove  as  satisfactory  as  any  method  for  temper- 
atures up  to  about  500°  F.  or  even  higher.  The  chief  requisites  are 
a  tank  holding  an  ample  supply  of  oil,  a  suitable  furnace  or  method 
of  heating  by  which  accurate  and  constant  temperatures  may  be 
obtained,  and  a  mercury  thermometer  for  determining  the  tempera- 
ture of  the  oil.  Mineral  oil  with  a  flash-point  of  some  600°  F.  is 


FIG,  59,— Quenching  and  Tempering  Dies,     ("  Machinery.") 


generally  used  for  the  bath ;  certain  of  the  animal  and  vegetable  oils 
are  also  occasionally  used. 

Handling  the  Material. — Oil  baths,  and  similarly  the  salt 
baths,  are  provided  with  a  wire  basket  in  which  the  pieces  to 
be  tempered  are  placed  and  which  is  then  lowered  into  the  oil. 
By  this  method  a  number  of  pieces  may  be  tempered  at  once, 
besides  preventing  the  steel  from  coming  in  contact  with  the  sides 
or  bottom  of  the  tank,  which  is  apt  to  be  hotter  than  the  oil. 
It  is  advisable,  whenever  possible,  to  allow  the  hardened  steel  to 


102  STEEL  AND  ITS  HEAT  TREATMENT 

come  up  gradually  to  the  desired  temperature,  and  not  to  immerse 
in  the  oil  when  the  latter  is  already  at  the  highest  heat.  Rather 
put  the  steel  in  the  oil  when  the  latter  is  about  200°  to  300°  F.  and 
let  the  two  heat  up  together.  The  reason  for  this  is  that  the  pre- 
heating— if  it  may  be  thus  termed— allows  the  heat  to  penetrate 
more  gradually,  softening  the  outer  portion  of  the  steel  in  such  a 
way  that  the  inner  and  stressed  part  may  be  more  gradually  relieved 
and  thus  avoiding  the  danger  of  fracture.  Sudden  heating  has  the 
tendency  to  set  up  new  stresses  which  must  in  turn  be  overcome. 
The  length  of  time  allowed  for  the  tempering  to  take  place  will 
depend  upon  the  size  and  nature  of  the  piece  under  treatment; 
fifteen  minutes  or  so  after  the  maximum  temperature  has  been 
reached  will  generally  be  sufficient  for  the  average  run  of  small 
tools,  gears,  etc.,  while  larger  parts  require  more  time  in  proportion. 
If  large  and  small  parts  are  tempered  at  the  same  time  it  will  do  no 
harm  to  the  small  pieces  if  they  are  not  removed  until  the  larger 
pieces  are  ready,  although  on  general  principles  long-continued 
heating  is  never  desirable  after  the  steel  has  responded  to  the  desired 
heating.  When  the  full  effect  of  the  tempering  has  been  attained, 
the  pieces  may  then  be  removed  from  the  oil  and  allowed  to  cool  off 
in  the  air,  for  if  the  steel  has  been  thoroughly  heated  at  the  maximum 
temperature  of  the  tempering  operation,  no  further  change  will 
take  place  in  the  ordinary  steels;  each  phase  of  the  transition  is 
represented  by  a  definite  temperature  for  each  steel,  so  that  no 
further  step  in  the  transition  will  occur  unless  the  temperature  is 
raised — with  the  possible  theoretic  exception  of  very  long-continued 
heating.  For  some  large  work,  such  as  die  blocks,  large  cutters, 
etc.,  the  steel  is  allowed  to  cool  off  in  the  oil  in  order  to  procure  the 
greatest  elimination  of  strains. 

Salt  Baths. — If  higher  drawing  temperatures  than  those  possible 
with  oil  are  desired,  a  bath  of  salts  may  be  used.  A  combination  of 
two  parts  of  potassium  nitrate  and  three  parts  of  sodium  nitrate 
melts  at  about  450°  F.  and  may  be  used  up  to  about  1000°  F. 
Methods  of  heating  and  using  are  similar  to  those  with  oil  baths,  and 
described  under  Hardening  Baths.  The  use  of  nitrate  salts  instead 
of  the  chloride  salts  is  necessary  on  account  of  the  lower  tempera- 
ture desired. 

Lead  Baths;  Alloys. — Lead,  having  a  melting-point  of  about 
610°  to  630°  F.,  may  also  be  used  for  tempering  where  temperatures 
higher  than  its  melting-point  are  required.  The  disadvantages  are 
similar  to  those  noted  under  its  use  for  heating  for  hardening.  The 


TEMPERING  AND  TOUGHENING 


103 


melting-point  may  be  lowered  by  alloying  the  lead  with  tin,  and 
temperatures  suitable  for  ordinary  tempering  may  be  obtained 
approximately  as  follows: l 


Lead 
Parts. 

Tin 
Parts. 

Approx.    Melt- 
ing Temp.  °  F. 

Lead 
Parts. 

Tin 
Parts. 

Approx.    Melt- 
ing Temp.  °  F. 

14 

8 

420 

28 

8 

490 

15 

8 

430 

38 

8 

510 

16 

8 

440 

60 

8 

530 

17 

8 

450 

96 

8 

550 

18  5 

8 

460 

200 

8 

560 

20 

8 

470 

Melted  lead 

610  to  630 

24 

8 

480 

The  use  of  these  various  alloys  of  predetermined  melting-points 
for  tempering  is  similar  to  that  previously  explained  when  selecting 
a  combination  of  salts  with  certain  melting-point  in  the  hardening 
operation. 

TOUGHENING 

Sorbite. — As  the  reheating  or  drawing  temperature  is  increased 
still  further  beyond  the  tempering  range  we  find  that  another  stage 
in  the  austenitic  transition  commences — the  change  of  troostite 
into  sorbite.  Like  the  change  from  martensite  to  troostite,  the 
formation  of  sorbite  does  not  take  place  spontaneously  throughout 
the  whole  steel,  but  increases  gradually  and  progressively.  Most 
writers  believe  that  sorbite  is  essentially  an  uncoagulated  conglomer- 
ate of  irresoluble  pearlite  with  ferrite  in  hypo-eutectoid  (less  than 
about  0.85  per  cent,  carbon),  and  cementite  in  hyper-eutectoid  steels 
respectively,  but  that  it  often  contains  some  incompletely  trans- 
formed matter.  Its  components  at  all  times  tend  to  coagulate 
into  pearlite.  On  higher  heating,  sorbite  changes  into  sorbitic 
pearlite,  then  slowly  into  granular  pearlite,  and  probably  indirectly 
into  lameliar  pearlite.  Sorbite  differs  from  troostite  in  that  it  is 
softer  for  a  given  carbon  content,  and  in  usually  being  associated 
with  pearlite  instead  of  martensite,  and  from  pearlite  in  being 
irresoluble  into  separate  particles  of  ferrite  and  cementite. 

Importance  of  Sorbite. — The  main  importance  of  sorbite  is  due 
to  its  physical  properties.  Although  slightly  less  ductile  than  pearl- 
itic  steel  for  a  given  carbon  content,  its  tenacity  and  elastic  limit  are 
so  high  that  a  higher  combination  of  these  three  properties  can  be 

1  Table  by  O.  M.  Becker,  using  melting-point  of  lead  as  610°  F. 


104  STEEL  AND   ITS  HEAT  TREATMENT 

had  in  sorbitic  than  in  pearlitic  steels.  Steels  which  are  so  treated 
as  to  contain  sorbite  are  often  called  "  toughened  "  steel. 

Toughening  Range.— The  transition  of  troostite— the  chief 
characteristic  of  tempered  steel,  into  sorbite — characteristic  of 
toughened  steel,  is  gradual,  and  progresses  with  the  increase  and 
duration  of  the  reheating.  At  some  point,  depending  upon  the 
composition  of  the  steel  and  the  degree  to  which  the  steel  has  been 
affected  by  the  hardening  process,  sorbite  is  formed.  If  we  accept 
sorbite  as  the  characteristic  constituent  of  toughened  steel  (and 
which  it  undoubtedly  is),  we  may  then  consider  as  the  lower  limit 
of  the  toughening  range  that  temperature  which  will  produce  sor- 
bite. In  fully  hardened  steel  of  the  medium  forging  and  higher 
carbon  analyses,  characteristic  sorbite  begins  to  form  at  about 
750°  F.  At  about  1250°  to  1300°  F.  the  sorbite  coagulates  into 
pearlite,  which  is  distinctive  of  annealed  steel.  With  these  facts 
in  view  we  may  then  consider,  in  a  general  way,  that  the  toughening 
range  lays  approximately  between  750°  and  1250°  F.  It  must  be 
remembered,  nevertheless,  that  these  temperatures  are  in  no  sense 
definite,  but  are  arbitrarily  taken  as  representative  of  a  class  of  heat- 
treatment  work:  differences  in  chemical  composition,  the  degree  of 
hardening,  the  size  of  work,  etc.,  all  play  their  part. 

Influence  of  Toughening. — When  a  piece  of  hardened  steel  is 
reheated  for  toughening,  each  specific  temperature  has  a  certain 
definite  influence  upon  the  steel.  The  results  of  this  toughening 
process  are  interpreted  by  the  ability  of  the  steel  to  do  certain  work, 
to  withstand  the  application  of  stated  loads,  or  as  measured  by 
standard  methods  of  testing.  On  account  of  the  almost  universal 
use  of  the  last  named  for  purposes  of  comparison,  we  will  deal  briefly 
with  (1)  the  static  strength,  as  measured  by  the  tensile  strength 
and  elastic  limit,  (2)  the  ductility,  as  measured  by  the  percentage 
elongation  and  reduction  of  area,  and  (3)  the  dynamic  strength,  as 
measured  by  the  alternating  impact  test. 

Effect  of  Increased  Temperature. — Each  increase  in  the  toughen- 
ing temperature  lowers  the  tensile  strength  and  elastic  limit,  but  with 
a  corresponding  increase  in  the  ductility  and  dynamic  endurance. 
With  the  majority  of  ordinary  carbon,  nickel,  chrome  and  vanadium 
steels  the  ratio  of  the  elastic  limit  to  the  tensile  strength  remains 
very  nearly  constant  throughout  the  sorbitic  range  (which  we 
assumed  to  be  approximately  from  750°  to  about  1250°  F.).  Be- 
yond these  temperatures,  and  coincident  with  the  formation  of  pearl- 
ite, the  values  for  the  elastic  limit  and  tensile  strength — of  each  par- 


TEMPERING   AND   TOUGHENING 


105 


106  STEEL  AND   ITS  HEAT  TREATMENT 

ticular  steel — begin  noticeably  to  diverge  until  they  reach  their 
smallest  ratio  in  fully  annealed  steel.  Up  to  near  the  end  of  the 
sorbitic  range  the  graphs  obtained  by  plotting  the  elastic  limit  and 
tensile  strength  against  the  drawing  temperatures  are,  for  general 
purposes,  straight  lines,  but  beyond  this  range  curve  towards  the 
horizontal,  as  is  represented  in  Fig.  60. 

Effect  on  Ductility. — These  changes  are  accompanied  by  reverse 
changes  in  the  ductility,  as  measured  by  the  reduction  of  area 
and  elongation.  As  interpreted  by  the  research  work  of  others, 
and  from  his  own  experimental  work,  the  author  is  inclined  to  the 
belief  that  these  two  factors  differ  from  each  other  in  that  the 
reduction  of  area  generally  reaches  a  maximum  at  about  the  end  of 
the  sorbitic  range  and  then  decreases,  while  the  elongation  does  not 
attain  its  maximum  until  the  steel  is  fully  annealed  or  in  the  pearlitic 
condition.  Be  this  as  it  may,  through  the  sorbitic  stage  at  least, 
each  increment  of  decrease  in  tensile  strength  and  elastic  limit  is 
associated  with,  and  counterbalanced  by,  an  increase  in  the  reduction 
of  area  and  elongation.  This  combination  of  static  strength  and 
ductility  is  further  almost  directly  proportional  to  the  toughening 
temperature. 

Impact  Strength. — The  effect  of  toughening  upon  other  properties 
and  especially  in  relation  to  the  impact  strength,  is  shown  in  Fig. 
60,  rearranged  from  the  work  of  Grard.  The  steels,  approximating 
0.15,  0.40  and  0.50  per  cent,  carbon,  were  hardened  and  then  re- 
heated to  temperatures  varying  from  no  tempering  up  to  2200°  F. 
The  impact  strength  curves  present  some  extremely  interesting  facts. 
We  find  that  the  greatest  resistance  to  shock  to  be  obtained  from 
a  toughening,  after  hardening,  at  a  temperature  about  100°  F. 
below  the  upper  critical  range  (Ac3);  annealing  at  a  temperature 
superior  to  the  Ac3  range  gives  a  lower  impact  strength.  Further, 
as  the  temperature  is  raised  more  and  more  and  overheating  results, 
there  is  a  marked  diminution  in  the  impact  strength.  Increase  in 
the  carbon  content,  assuming  the  same  heat  treatment,  diminishes 
the  impact  strength.  Tempering  (reheating  up  to  say  600°  F.) 
has  little  or  no  effect  upon  the  impact  strength.  As  a  general  prop- 
osition we  may  sum  up  by  stating  that  it  is  preferable,  in  order 
to  obtain  the  greatest  impact  strength,  to  keep  the  'carbon 
content  as  low  as  possible  and  to  have  a  high  drawing  tempe- 
rature. 

Capacity  of  the  Steel. — Thus  it  will  be  seen  that  by  changing  the 
drawing  temperature  the  grouping  of  these  factors  may  be  varied 


TEMPERING  AND   TOUGHENING  107 

through  a  considerable  range  and  limited  only  by  what  we  may  call, 
for  want  of  a  better  phrase,  the  "  capacity  of  the  steel."  This 
quantity  is  defined  largely  by  the  chemical  composition,  the  method 
of  manufacture,  the  size  of  the  piece  to  be  treated,  and  by  other 
subordinate  factors.  With  these  qualifying  conditions  in  mind,  we 
may  further  define  the  capacity  of  the  steel  as  the  limiting  ratio  of 
strength  to  ductility.  Each  steel,  as  qualified  above,  has  certain 
definite  limits  within  which  the  physical  properties  may  be  varied. 
At  one  end  of  the  see-saw,  as  in  hardened  steel,  there  is  a  maximum 
tensile  strength  with  minimum  ductility;  and  at  the  other  extreme, 
as  in  fully  annealed  or  sorbitic-pearlitic  steel,  there  will  be  a  mini- 
mum tensile  strength  with  maximum  ductility.  Following  out  the 
simile  of  the  see-saw,  we  may  place  tenacity  on  one  end  and  ductility 
on  the  other;  when  one  is  up,  the  other  must  be  down;  both  cannot 
be  up  nor  both  down  at  one  and  the  same  time;  raise  one  and  the 
other  must  fall.  The  heat-treatment  man  now  stands  on  the  middle 
of  the  board  and  by  means  of  his  reheating  temperature  can  adjust 
the  opposing  factors  to  that  position  which  he  desires;  but  he  cannot 
change  the  maximum  and  minimum  of  either,  because  they  are  fixed 
by  the  limitations  previously  mentioned  at  the  beginning  of  the 
paragraph,  and  over  these  he  has  no  control  as  far  as  the  individual 
stael  is  concerned. 

Duplication  of  Results. — Happily  for  the  heat-treatment  man, 
each  grouping  is  distinctive  of  a  definite  toughening  temperature, 
other  conditions  being  the  same.  When  he  has  once  determined  the 
relation  existing  between  static  strength,  ductility,  and  temperature, 
for  a  given  size  piece  of  work  made  from  a  steel  of  specific  analysis, 
he  knows  that  he  can  approximately  duplicate  his  results  under  like 
conditions  at  any  time.  Not  that  he  can  absolutely  and  ultra- 
scientifically  obtain  results  within  a  few  pounds  elastic  limit  or  hun- 
dredths  of  a  per  cent,  elongation — for  such  are  neither  necessary 
nor  expected — but  that  he  can  reasonably  expect  to  get  a  com- 
mercially acceptable  duplication.  It  is  with  this  thought  in  mind 
that  the  subsequent  chapters  have  been  developed,  giving  under 
each  steel  many  of  the  results  and  details  which  have  been  obtained 
in  practice  and  experiment,  and  which  should  prove  advantageous 
to  the  average  heat-treatment  man  as  a  time-saver. 

Slow  Cooling  and  Stresses  and  Strains. — It  is  one  of  the  incon- 
trovertible facts  of  heat-treatment  work  that  slow  cooling  predicates 
the  release  of  internal  stresses  and  strains.  Not  only  is  this  true 
of  the  full-annealing  process — as  indicative  of  slow  cooling  from  a 


108  STEEL  AND   ITS  HEAT  TREATMENT 

temperature  above  that  of  the  critical  range,  but  also  of  the  toughen- 
ing operation.  In  fact,  the  very  nature  of  the  usefulness  of  tough- 
ened steel  depends  upon  the  absence  of  a  state  of  strain  just  as  much 
as  upon  specific  static  or  dynamic  properties.  Strange  as  it  may 
seem,  some  of  the  failures  in  locomotive  forgings  may  be  traced 
back  to  the  lack  of  slow  cooling  after  toughening;  and  this  trouble 
is  coming  to  be  recognized  in  many  specifications  by  the  require- 
ment of  cooling  in  the  furnace  after  toughening.  Just  as  the  dangers 
in  hardening  increase  with  the  rapidity  of  cooling,  carbon  content 
and  size  of  section,  so  are  they  likewise  magnified  in  cooling  after 
toughening — although  on  a  smaller  scale.  If  these  factors  become 
noticeably  important,  cooling  in  air  from  the  toughening  tempera- 
ture may  set  up  such  a  new  series  of  cooling  strains  that  many  of 
the  real  advantages  of  toughening  may  be  invalidated. 

Use  of  Furnace  Cooling. — The  greater  part  of  hardened  and 
toughened  work,  such  as  automobile  and  other  small  forgings,  may 
not  require  furnace  cooling,  besides  being  economically  impracticable. 
But  even  with  these  it  is  desirable  that  the  pieces  should  be  piled 
together  after  removal  from  the  furnace  so  that  the  cooling  will  be 
retarded.  For  forgings  of  section  greater  than  3  or  4  ins.,  such  as 
heavy  machine  parts,  ordnance,  etc.,  cooling  in  the  furnace  is 
always  desirable.  It  may  be  said  that  such  slow  cooling  never  did 
any  harm,  and  it  may  do  a  world  of  good  in  relieving  strains. 

Effect  of  Furnace  Cooling  on  Physical  Properties. — Contrary  to 
the  opinion  held  by  some,  the  author  does  not  believe  that  slow 
cooling  in  the  furnace  has  any  noticeable  tendency  to  further 
"  soften  "  the  usual  straight  carbon  or  alloy  steels  to  which  the 
toughening  process  is  generally  applied.  That  is,  for  similar  pieces 
of  the  same  steel  treated  alike,  equivalent  physical  test  results  would 
be  obtained  in  the  forging  which  had  been  furnace  cooled  as  in  the 
one  which  had  been  allowed  to  cool  in  the  air — the  tests  being  taken 
from  the  same  relative  position.  In  making  this  statement  there 
is,  however,  one  other  necessary  qualification:  it  is  assumed  that 
the  whole  mass  of  the  steel  has  been  thoroughly  heated  at  the  tough- 
ening temperature.  Otherwise  the  effect  of  the  toughening  would 
not  be  so  great  in  the  air-cooled  piece  as  in  the  slowly  cooled  piece, 
for  the  latter  would  have  greater  opportunity  to  be  affected  by  the 
heat  of  the  furnace  during  the  furnace  cooling.  During  the  tough- 
ening range  the  effect  of  the  heat  upon  the  transition,  except  for 
very  large  pieces,  practically  ceases  as  soon  as  the  source  of  heat  is 
removed — as  by  air  cooling. 


TEMPERING  AND   TOUGHENING  109 

High  vs.  Low  Toughening  Temperatures. — On  the  hypothesis 
that  either  of  two  specified  analyses  would  prove  equally  satisfactory, 
under  suitable  treatment,  for  the  same  piece  of  work,  but  that  on 
account  of  the  difference  in  chemical  composition  one  steel  would 
require  toughening  at  say  1200°  F.  and  the  other  at  say  800°  or 
900°  F.,  the  selection  of  the  higher  drawing-point  steel  should  be 
ma/le.  Such  conditions  often  arise  in  heat-treatment  plants  handling 
a  variety  of  commercial  work  and  it  may  be  well  to  sum  up  briefly 
the  reasons  for  the  above  conclusion. 

The  more  stable  the  state  of  equilibrium  which  exists  between 
the  transition  constituents  the  more  lasting  and  effectual  will  be  the 
treatment.  Further,  the  smaller  the  amount  of  internal  strains 
which  may  remain  in  the  steel  from  the  previous  hardening  operation 
the  better.  Both  of  these  conditions  are  more  nearly  brought  about 
by  the  higher  drawing  temperature. 

As  there  is  also  a  decided  tendency  for  the  dynamic  strength 
to  reach  a  maximum  at  about  1200°  to  1300°  F.  it  is  probable  that 
the  higher  drawing  temperature  steel  will  have  a  greater  dynamic 
strength  than  the  other  steel,  provided  that  there  is  not  too  much 
difference  between  the  chemical  compositions  of  the  two  steels. 

From  the  furnace  man's  point  of  view  the  temperatures  around 
1200°,  being  of  characteristic  visible  reds,  are  decidedly  more  easily 
recognizable  than  those  temperatures  around  800°  to  1000°,  since 
with  these  lower  temperatures  there  is  very  little  visible  heat  color. 
The  higher  drawing  temperatures  therefore  aid  in  the  efficiency  of 
judging  the  heating  operation  and  lead  to  greater  uniformity  of 
control  and  of  results. 

Quenching  Medium  vs.  Toughening  Temperature. — There  is 
another  phase  of  the  high  or  low  toughening  temperature  proposition 
which  cannot  be  solved  by  any  general  rule,  but  only  after  due 
consideration  of  all  the  circumstances  involved;  this  relates  to  the 
condition  of  affairs  when  there  is  no  opportunity  for  the  choice  of 
steel,  but  depends  more  upon  the  selection  of  the  quenching  medium 
in  relation  to  the  toughening  temperature.  As  we  have  noted, 
water  quenching  gives  a  harder  steel  than  oil  quenching.  It  natu- 
rally follows  that,  in  order  to  obtain  approximately  the  same  physi- 
cal results,  the  oil-quenched  piece  must  be  drawn  at  a  lower  temper- 
ature than  the  water-quenched  piece.  The  arguments  regarding 
water  vs.  oil  quenching,  and  low  vs.  high  drawing  temperatures  have 
been  previously  discussed.  If  the  solution  were  to  be  developed 
entirely  along  these  lines  it  is  orobable  that  in  the  majority  of  cases; 


110  STEEL  AND   ITS  HEAT  TREATMENT 

the  oil  quenching  (giving  less  hardening  strains)  and  lower  drawing 
temperature  would  be  employed.  In  other  words,  the  difficulties 
to  be  encountered  with  water  quenching — the  hardening  operation 
being  the  more  drastic  of  the  two — would  more  than  outweigh  the 
the  disadvantages  of  the  lower  toughening  temperature.  This  is  a 
question  in  which  the  personal  element  and  experience  of  the  heat- 
treatment  man  would  be  paramount. 

Influence  of  the  Carbon  Content. — In  respect  to  the  selection  of 
the  steel  in  relation  to  the  treatment  there  remains  the  consideration 
of  the  influence  of  the  carbon  content.  Carbon  not  only  intensifies 
the  effect  of  the  rapid  cooling  (hardening),  but  it  also  directly 
augments  the  brittleness  of  the  steel.  Or,  to  put  it  in  other  words, 
the  greater  the  carbon  content  the  greater  the  hardening  strains, 
and  the  lower  the  ductility  which  can  be  obtained  with  a  stated 
tensile  strength.  It  is  therefore  usually  desirable  to  provide  a  steel 
with  as  low  a  carbon  content  as  will  give  the  desired  results. 

Toughening  vs.  Annealing. — It  is  only  within  comparatively 
recent  years  that  the  toughening  process  with  its  attendant  sorbitic 
structure  has  been  used  and  understood.  Previously,  annealing 
was  generally  the  cure-all  for  brittleness  and  a  strained  condition 
of  the  steel.  Pearlite — produced  by  annealing — on  account  of  its 
entangled  structure,  gives  a  large  measure  of  ductility;  but  also 
gives  a  minimum  tenacity.  The  appearance  of  sorbite,  however,  is 
even  more  entangled  than  pearlite;  sorbite  is  far  superior  to  pearlite 
in  tensile  strength  and  especially  in  elastic  limit.  Thus  by  obtaining 
a  sorbitic  steel  by  suitable  treatment,  almost  as  much  ductility, 
greater  working  strength,  greater  dynamic  strength,  and — by  being 
able  to  use  a  lower  carbon  steel — less  brittleness  may  be  obtained 
than  in  a  pearlitic  or  annealed  steel. 

Standardization  of  Results. — With  the  same  degree  of  hardening, 
and  if  the  reheating  has  been  uniiorm  and  thorough  at  a  given  tem- 
perature, the  physical  results  will  be  comparatively  the  same  for 
material  of  equivalent  section  and  the  same  composition.  That 
is,  the  product  will  be  standard  for  standardized  treatment.  Fur- 
ther, in  order  to  get  standard  results  with  steel  purchased  under  the 
same  general  specifications  (i.e.,  each  chemical  constituent  within 
certain  limits) ,  the  toughening  temperatures  may  be  varied  according 
to  the  chemical  composition.  To  illustrate:  the  following  heats  of 
steel  of  varying  chemical  composition  and  made  by  several  steel 
companies  were  manufactured  into  a  certain  product  which,  when 
heat  treated,  required  an  elastic  limit  of  85,000  to  95,000  Ibs.  per 


TEMPERING  AND   TOUGHENING 


111 


square  inch,  and  an  elongation  of  not  less  than  16  per  cent,  in 
2  ins.  In  spite  of  the  varying  carbon,  manganese,  chrome  and 
nickel  contents,  the  toughening  temperatures  (maintained  within 
5°  F.  under  or  over)  were  so  adjusted  as  to  give  the  desired  results. 
Thousands  of  pieces,  some  weighing  as  much  as  200  Ibs.,  all  ful- 
filled, by  actual  test,  the  standard  physical  specifications. 


Carbon. 

Manga- 
nese. 

Phosphorus. 

Sulphur. 

Chrome. 

Nickel. 

Toughen- 
ing Temp. 
Deg.  Fahr. 

0.16 

0.43 

0.015 

0.017 

0.62 

1.82 

1050 

.185 

.44 

.010 

.015 

.57 

1.56 

975 

.20 

.43 

.009 

.016 

.64 

1.74 

1025 

.20 

.46 

.011 

.017 

.40 

1.56 

950 

.21 

.48 

.015 

.015 

.60 

1.77 

1050 

.21 

.50 

.017 

.014 

.67 

1.84 

1075 

.23 

.50 

.016 

.018 

.65 

1.73 

1075 

.245 

.53 

.015 

.020 

.62 

1.79 

1120 

.25 

.50 

.011 

.019 

.64 

1.40 

1140 

.26 

.43 

.010 

.018 

.60 

1.65 

1100 

27 

.49 

.015 

.021 

.63 

1.79 

1150 

.28 

.51 

.008 

.011 

.41 

1.57 

1120 

Quench-Toughening.— A  process  which  has  been  used  consider- 
ably for  the  treatment  of  large  forgings  of  uniform  section,  such  as 
heavy  axles,  is  that  of  heating  as  usual  for  hardening  and  then 
quenching  in  oil  for  a  specified  number  of  seconds,  followed  by  air 
cooling.  The  oil  quenching  affects  the  steel  to  a  certain  depth,  but 
still  leaves  a  considerable  amount  of  heat  in  the  forging  when 
removed  from  the  bath.  As  the  forging  cools  in  the  air  this  heat 
from  within  will  toughen  or  "  soften  "  the  steel  affected  by  the 
quenching.  In  order  to  obtain  equivalent  results  under  varying 
conditions  the  number  of  seconds  required  for  immersion  in  the  oil 
of  a  piece  of  given  size  must  be  determined  by  experiment  and  strictly 
adhered  to.  Forgings  treated  by  this  process  are  characterized  by 
a  soft  or  annealed  core,  with  a  progressively  toughened  outer  part. 

Physical  Results. — In  subsequent  chapters  will  be  given  results 
obtained  in  actual  practice  by  the  use  of  various  toughening  tem- 
peratures for  different  grades  of  steel, 


CHAPTER    VI1 
CASE   CARBURIZING 

Object  of  Case  Hardening. — The  object  of  case  hardening  or 
partial  cementation  is  the  production  of  a  hard  wearing  surface  (the 
"  case  ")  on  low  carbon  steel,  and  at  the  same  time  the  retention  or 
increase  of  the  toughness  of  the  "  core  "  of  the  metal.  The  process 
may  be  roughly  divided  into  two  distinct  periods.  First,  the  car- 
burization  or  impregnation  of  the  surface  by  which  the  carbon  con- 
tent is  sufficiently  raised — dependent  upon  the  demands  of  the  work 
— so  as  to  give  a  steel  capable  of  taking  on  very  great  surface  hardness. 
Second,  suitable  heat  treatment  which  shall  develop  the  properties 
of  both  case  and  core.  The  complete  operation  should  not  only 
result  in  the  obtaining  of  a  very  hard  case,  but  also  and  simultaneously 
in  the  realization  of  special  mechanical  properties  in  the  core — more 
especially  that  of  non-brittleness.  Briefly,  the  aim  is  to  have  a 
piece  of  steel  which  shall  possess  a  minimum  fragility  and  a  maximum 
surface  hardness. 

Requirements  for  Case  Carburizing. — In  order  to  obtain  a  case 
rich  in  carbon,  the  metal  is  heated  in  the  presence  of  a  body  which  is 
capable  of  delivering  this  carbon,  by  more  or  less  complex  reactions, 
which  is  then  dissolved  by  the  steel.  Aside  from  the  use  of  gases 
in  the  newer  processes  involving  such  factors  as  pressure,  quantity, 
etc.,  there  are  four  main  factors  which  must  be  considered  in  the 
carburizing  operation: 

1.  The  solvent:  that  is,  the  steel; 

2.  The  product  to  be  dissolved,  or  more  exactly,  the  compound 

capable  of  delivering  the  carbon,  i.e.,  the  cement; 

3.  The  temperature; 

4.  The  time  of  contact  between  the  steel  and  the  carburizing 

agent. 

1Cuts  by  Giolitti  from  "  The  Cementation  of  Iron  and  Steel,"  by  courtesy  of 
McGraw-Hill  Book  Co.;  references  made  in  this  chapter  to  investigations  by 
Giolitti  are  also  from  the  above. 

112 


CASE  CARBURIZING  113 


THE    STEEL 

The  Steel. — The  character  of  the  initial  steel  used  for  case  car- 
burizing  depends  largely  upon  the  fact  that  one  of  the  main  desires 
is  to  eliminate  brittleness  in  the  core.  We  have  seen  that  any 
increase  of  carbon,  other  conditions  being  equal,  will  increase  the 
brittleness,  particularly  when  the  carbon  content  is  raised  to  over 
about  0.25  per  cent.  Further,  as  practically  all  commercial  car- 
burizing  processes  involving  case  hardening  are  followed  by  one  or 
more  hardening  operations,  it  follows  that  the  use  of  a  steel  with  a 
higher  carbon  content  will  also  increase  the  brittleness  through 
quenching.  For  these  reasons  it  is  therefore  necessary  to  keep 
the  carbon  content  of  the  steel  to  be  carburized  quite  low,  prefer- 
ably under  0.25  per  cent,  for  straight  carbon  steels.  In  fact,  the  best 
French  practice  is  to  demand  a  carbon  content  of  not  over  0.12  per 
cent.,  further  qualified  by  the  specifications  that  the  core  after 
quenching  shall  give  a  tensile  strength  of  about  54,000  Ibs.  per  square 
inch  and  not  to  exceed  85,000  Ibs.,  together  with  an  elongation  of 
30  per  cent,  in  100  mm.  (3.94  ins.). 

However,  one  of  the  important  and  often  unsatisfactory  results 
of  using  an  extra-soft  steel  is  the  difficulty  encountered  in  machining 
(before  carburizing) .  If  the  carbon  is  extremely  low  the  steel  is 
very  apt  to  tear,  and  thus  increasing  the  amount  of  grinding  after 
hardening — in  order  to  obtain  a  perfectly  smooth  surface.  For 
this  reason,  the  general  American  practice  is  to  adopt  a  carbon 
content  about  midway  between  the  extreme  upper  and  lower  limits 
and  specify  a  steel  with  about  0.16  to  0.22  per  cent,  carbon.  The 
higher  carbons  also  give  increased  stiffness  to  the  core  which,  in 
some  cases,  is  necessary. 

It  is  generally  recognized  that  the  carbon  content,  at  least 
up  to  some  0.50  per  cent.,  has  no  influence  upon  the  velocity  of 
penetration  of  the  carburization,  i.e.,  the  depth  of  carburization 
which  will  be  obtained  for  a  given  length  of  exposure. 

On  the  other  hand,  the  initial  carbon  content  of  the  steel  will 
have  a  decided  influence  upon  the  maximum  carbon  content  which 
will  be  obtained  in  the  case;  the  higher  the  initial  carbon,  the  higher 
the  maximum  carbon  concentration  in  the  case. 

Manganese. — It  is  considered  the  best  practice,  in  general,  to 
require  a  low  manganese  content  with  about  0.30  to  0.35  per  cent, 
as  the  maximum.  It  should  be  remembered  that  the  case  which 
will  be  formed  during  the  carburization  will  be  characteristic  of 


114  STEEL  AND  ITS  HEAT  TREATMENT 

a  high-duty  tool  steel  and  will  have  the  properties  of  such.  Thus 
manganese  will  increase  the  hardness  of  the  case  (and  also  of  the 
core)  and  will  make  the  steel  as  a  whole  more  sensitive  to  rapid  cool- 
ing. In  spite  of  this,  it  is  often  customary,  especially  in  British 
practice,  to  use  a  manganese  content  of  about  0.70  per  cent. — and 
in  some  cases  even  up  to  0.90  per  cent. — in  order  to  obtain  greater 
stiffness  in  the  core.  Manganese  at  such  percentages  also  increases 
the  brittleness  produced  by  long  heating  during  carburization,  and 
diminishes  the  efficacy  of  the  regenerative  quenching.  These  last 
named  points  are  also  true  when  the  silicon  is  much  over  0.30  per 
cent. 

Other  Impurities. — It  is  self-evident  that  the  content  of  phos- 
phorus and  sulphur  in  the  initial  steel  should  be  just  as  low  as  is 
possible.  Slag,  blow-holes,  segregation,  and  all  other  impurities 
and  imperfections  should  be  entirely  absent  from  steels  for  case 
hardening. 

THE  CEMENT 

Direct  Action  of  Carbon. — Carburization  by  its  very  nature 
requires  the  presence  of  free  carbon  in  some  form  or  other,  either  as 
a  solid  body,  or  as  some  gas  which  will  produce  free  carbon  by  its 
decomposition.  The  mere  presence  of  free  carbon  in  contact  with 
iron,  however,  will  not  satisfy  the  conditions  necessary  for  commercial 
carburization.  Although  it  has  been  shown  scientifically  that  car- 
bon alone,  without  the  intervention  of  any  gas,  will  carburize  iron 
if  it  is  kept  in  contact  with  it  for  a  sufficiently  long  time  and  at  a 
sufficiently  high  temperature,  this  direct  action,  as  far  as  industrial 
results  are  concerned,  is  negligible.  That  is,  the  ordinary  forms 
of  solid  carbon,  such  as  wood  charcoal,  sugar  charcoal,  etc.,  exercise 
directly  on  iron  but  a  very  slight  carburizing  action  in  the  absence  of 
gases. 

Action  of  Gases. — It  will  be  noted  that  emphasis  has  been  laid 
upon  the  "  direct  action  "  in  the  "  absence  of  gases."  This  at  once 
leads  to  the  question  as  to  what  is  meant  by  the  action  of  gases,  and 
which,  in  turn,  involves  the  mechanism  of  cementation  itself.  It  is 
a  well-known  fact  that  when  steel  is  heated,  the  "  pores  of  the  steel 
are  opened  " — to  use  the  vernacular  expression — it  becomes  easily 
permeable  to  gases,  and  the  surrounding  gases  diffuse  into  the  steel. 
This  is  true  whether  the  steel  is  heated  in  the  ordinary  atmosphere, 
when  the  gases  consist  of  nitrogen  and  oxygen,  or  whether  it  is  heated 
in  some  specially  prepared  atmosphere,  such  as  carbon  monoxide, 


CASE  CARBURIZING  115 

illuminating  gas,  etc.  The  main  fact  to  be  realized  is  that  the  gases 
do  penetrate  into  the  steel,  although  the  effect  of  the  gases  will 
depend  upon  the  composition  of  the  gas,  besides  such  other  factors 
as  pressure,  temperature,  and  so  forth.  Thus,  recognizing  that  the 
direct  action  of  carbon — that  is,  the  carburizing  results  obtained  by 
mere  contact  of  carbon  with  iron — is  commercially  negligible  in  the 
absence  of  gases,  it  is  evident  that  carburization  must  be  intimately 
related  to  the  presence  of  gases.  In  other  words,  the  gases  (or,  more 
exactly,  certain  gases)  must  in  themselves  act  as  the  carrier  or 
vehicle  for  the  carbon.  That  this  carrier  action,  or  transporting  of 
the  carbon,  has  not  been  definitely  recognized  or  determined  until 
recently  has  been  due  to  the  fact  that  practically  all  of  the  solid 
cements  generate  the  necessary  gases  through  their  own  decomposi- 
tion and  interaction  with  the  occluded  air.  Further,  the  intense 
and  critical  study  of  this  action  has  been  developed  only  by  the 
research  work  in  connection  with  the  newer  processes  of  case 
carburizing  by  means  of  gases  alone. 

Action  of  Oxygen. — As  a  typic  1  example  of  this  diffusion  and  its 
effect  we  may  consider  any  ordinary  carburization  process  in  which 
wood  charcoal  is  used  as  the  base  cement.  When  the  carburizing 
material  and  articles  are  packed  in  the  carburization  boxes  there  is 
necessarily  a  considerable  quantity  of  air  also  occluded  with  the 
particles  of  the  cement.  Under  the  influence  of  heat  the  oxygen  of 
the  occluded  air  will  react  with  the  carbon  or  charcoal  to  form  car- 
bon monoxide  gas,  which  has  the  symbol  CO.  Then,  as  the  tem- 
perature of  the  box  and  contents  increases  to  the  temperature  of  the 
carburization  proper,  these  gases  of  carbon  monoxide  permeate  or 
diffuse  through  the  surface  and  outer  section  of  the  steel.  At  the 
same  time,  by  catalytic  action,  the  carbon  monoxide  gas  decom- 
poses when  it  comes  in  contact  with  the  steel  and  sets  free  a  part 
of  the  carbon  it  contains.  This  decomposition  may  be  represented 
by  the  reversible  reaction 

2CO  <±  C02  +  C 

carbon  monoxide^carbon  dioxide  (gas)  -+-  carbon  (solid). 

Thus,  as  the  gas  diffuses  into  the  mass  of  the  steel  it  continues  to 
decompose,  setting  free  new  quantities  of  carbon  within  the  steel. 
This  carbon,  at  the  proper  temperatures  of  carburization,  passes 
directly  into  solution  in  the  steel  and  forms  a  true  steel  proper.  The 
reaction  above,  being  reversible — as  might  be  shown — will  continue 
indefinitely  under  suitable  conditions,  the  charcoal  regenerating  the 


116  STEEL  AND   ITS  HEAT  TREATMENT 

supply  of  carbon  monoxide.  Further,  while  it  is  a  well-known  fact 
that  carbon  monoxide,  acting  alone  on  iron,  will  deposit  free  carbon 
on  the  surface  of  the  iron,  this  action  takes  place  only  at  temperatures 
lower  than  those  ordinarily  used  for  commercial  cementation.  In 
other  words,  the  carburizing  action  of  charcoal  as  used  in  practice 
is  not  due  to  the  direct  action  of  the  carbon,  but  is  due  (under  the 
conditions  named,  which  of  course  may  be  modified  by  the  presence 
of  other  gases  or  components  of  the  cement)  entirely  to  the  specific 
action  of  carbon  monoxide  as  a  gas. 

Nitrogen. — The  action  of  the  oxygen  of  the  occluded  air  being 
accounted  for,  the  accompanying  constituent  nitrogen  must  be  con- 
sidered. Although  it  has  been  shown  that  during  carburization  the 
nitrogen  may  and  will  diffuse  in  small  amounts  into  the  steel,  it  is 
now  certain  that  the  presence  of  pure  nitrogen  does  not  increase, 
except  to  a  minimum  extent,  the  carburizing  action  of  free  carbon. 
In  fact,  instead  of  nitrogen  being  requisite — as  many  still  believe — 
it  may  even  exert  a  pernicious  effect.  LeChatelier  has  suggested  that 
the  increase  in  brittleness  sometimes  observed  in  those  parts  of  the 
steel  subjected  to  cementation,  but  which  the  carburization  has  not 
even  reached,  may  be  due  to  this  nitrogen.  It  might  be  added  that 
this  deleterious  nitrogenizing  theory  is  further  supported  by  experi- 
ments along  other  lines — particularly  in  the  apparent  cleansing  effect 
for  nitrogen  of  the  titanium  additions  to  steel  during  manufacture. 
Another  general  effect  of  nitrogen  gas  is  to  reduce  the  cementing 
action  of  the  carbon  monoxide  mentioned  by  its  diluting  the  car- 
burizing gas.  For  practical  purposes  of  carburization,  however, 
the  action  of  nitrogen  in  the  presence  of  free  carbon  is  too  slight  to 
influence  commercially  the  results  obtained  with  a  given  cement, 
unless  actually  added  (in  gaseous  cementation)  as  a  diluent. 

Carbonates. — The  ash  of  the  carbonaceous  matter  may  also 
contain  carbonates  of  the  alkali  or  alkaline-earth  metals.  Or  these 
carbonates,  such  as  barium  carbonate,  may  be  added  directly  to  the 
cement.  In  the  light  of  the  most  reliable  and  recent  researches  it 
would  appear,  contrary  to  previously  accepted  theories,  that  the 
activity  of  these  carbonates  is  not  due  to  the  formation  of  volatile 
cyanides  by  the  action  of  the  nitrogen  of  the  occluded  air,  but  exclu- 
sively to  the  formation  of  carbon  monoxide  produced  by  the  action 
of  the  hot  carbon  on  the  carbon  dioxide  produced  through  the  dis- 
sociation of  the  carbonates.  Thus  the  effect  of  such  carbonates 
is  similar  to  that  produced  by  carbon  monoxide  under  similar  con- 
ditions. 


CASE  CARBURIZING  117 

Cyanides. — The  most  maligned  constituents  of  cements  are  the 
cyanogen  group.  In  the  past  it  has  been  thought  that  the  deriva- 
tives of  this  group  played  the  chief  part  in  carburization  processes. 
This,  however,  has  been  strongly  disproved  by  Giolitti,  who  ad- 
mirably explains  the  matter  as  follows:  That  cyanogen  and  the 
more  or  less  volatile  cyanides  can  cement  iron  intensely  is  beyond 
doubt.  Moreover,  it  is  well  known  that  fused  potassium  and  potas- 
sium ferrocyanide  are  used  in  the  pure  state  to  obtain  thin  and 
strongly  carburized  zones  (as  in  superficial  carburization  or  cyanide 
hardening) .  In  industrial  practice  the  cyanides  do  not  exist  already 
formed,  but  may  be  formed  in  very  small  quantity  by  the  action  of 
the  nitrogen  of  the  air  (occluded  in  the  cement)  on  the  carbon  used 
and  on  the  small  quantities  of  alkali  constituting  a  part  of  the  ashes 
of  this  carbon.  Although  the  formation  of  small  quantities  of  alkali 
cements  cannot  therefore  be  wholly  avoided  in  industrial  car- 
burization with  carbon  as  a  base,  the  part  which  is  played  by  these 
traces  of  volatile  cyanides  is  certainly  negligible  in  comparison  with 
that  of  the  carbon  monoxide  formed  by  the  action  of  the  air  on  the 
carbon  used  as  cement. — He  then  submits  conclusive  proofs  to 
substantiate  these  statements. 

Carbon  Monoxide  Gas. — Carburization  carried  out  by  the  use  of 
carbon  monoxide  gas  alone  will  give  a  mild  or  gradual  carburization 
in  which  the  maximum  carbon  content  is  comparatively  low — 
not  usually  reaching  the  eutectoid  ratio  even  at  the  periphery — and 
which  diminishes  progressively  and  in  a  uniform  and  slow  manner 
passing  from  the  surface  of  the  case  toward  the  interior  of  the  car- 
burized piece.  Carburized  zones  of  this  type  correspond  always 
and  only  to  carburization  carried  on  with  pure  carbon  monoxide, 
a  concentration-depth  diagram  of  which  is  shown  in  Fig.  61.  On 
account  of  its  definite  chemical  composition  and  simplicity  of  action, 
the  general  behavior  of  carbon  monoxide  is  known  within  almost 
exact  limits.  The  carburizing  action  is  easily  regulated,  and  the 
case  may  be  obtained  with  certainty  with  any  kind  of  steel  in  com- 
mercial use. 

When  working  under  suitable  conditions,  carbon  monoxide — 
either  alone  or  with  a  mixture  in  which  the  carbon  monoxide  can  exer- 
cise its  maximum  carburizing  action — will  give  the  greatest  velocity 
of  carburization,  i.e.,  the  depth  reached  in  a  given  time  by  the  car- 
burized zone.  This  depth  is  also  a  direct  function  of  the  time  or 
length  of  exposure. 

All  other  conditions  being  equal,  the  higher  the  temperature  of 


118 


STEEL  AND   ITS  HEAT  TREATMENT 


carburization  using  carbon  monoxide,  the  smaller  will  be  the  maxi- 
mum carbon  content  of  the  case.  Similarly,  the  lower  the  pressure 
of  the  carbon  monoxide,  the  smaller  the  maximum  carbon  content; 
and  the  greater  the  quantity  of  pure  carbon  monoxide  gas  coming 
in  contact  with  a  unit  of  surface,  the  greater  the  carbon  concentra- 
tion. 

Under  suitable  conditions,  carbon  monoxide  gas  will  deposit 
no  carbon  on  the  surface  of  the  steel  being  carburized,  so  that  there 
is  little  difficulty  in  keeping  the  surface  bright.  Further,  the  use  of 
carbon  monoxide  reduces  to  a  minimum  the  deformations  and  varia- 
tions in  volume  due  to  the  carburizing  processes.  Carbon  monoxide 
also  lends  itself  in  obtaining  a  good  protection  of  the  parts  of  the  steel 
which  it  is  not  desired  to  carburize. 


i.o 

0.8 
0.6 
0.4 
0.2 


0.5 


1.5 


2.5        3  MM. 


FIG.  61. — Carburization  at  2010°  F.  for  Ten  Hours  with  Carbon  Monoxide. 

(Giolitti.) 

Hydrocarbons. — Most  of  the  forms  of  solid  carbon  used  in  prac- 
tical carburization  are  not  pure,  but  may  contain  organic  residues  not 
wholly  decomposed,  or  considerable  proportions  of  ash  rich  in  cer- 
tain carbonates.  Thus  charred  bone,  charred  leather  and  similar 
organic  products  often  used,  will,  under  the  influence  of  heat,  evolve 
hydrocarbons.  These  hydrocarbons,  by  more  or  less  complex 
reactions,  deposit  the  excess  of  finely  divided  carbon  which  they  con- 
tain on  the  surface  of  the  metal;  and  this,  in  turn,  being  in  perfect 
contact  with  the  metal,  at  high  temperatures  may  cause  a  direct 
carburization  by  contact.  But  further  and  vastly  more  important 
than  this  direct  action  of  the  carbon  deposit  on  the  surface  of  the 
metal,  is  the  carburization  by  means  of  the  specific  action  of  the  gas 
itself,  although  of  course  depending  more  specifically  upon  the  exact 
conditions  of  carburization.  In  a  manner  somewhat  analogous  to 


CASE  CARBURIZING 


119 


that  of  the  decomposition  of  the  carbon  monoxide  within  the  steel, 
yielding  carbon  directly  to  the  steel,  the  hydrocarbon  gases  will  also 
diffuse  into  the  steel  and  there  yield  carbon.  Hydrocarbons  there- 
fore also  act  as  carriers  for  the  carbon  and  effect  a  carburization  due 
to  the  specific  action  of  the  gas. 

Carburization  with  pure  hydrocarbon  gases  give  cases  of  a  type 
corresponding  to  Figs.  62  and  63,  and  to  Fig.  64.  These  are 
characterized  on  slow  cooling  by  (1)  a  layer  or  zone  of  hyper-eutectoid 
steel  consisting  of  free  cementite  and  pear  lite ;  (2)  by  a  layer  of  eutec- 
toid  steel,  generally  quite  thin;  and  (3)  by  an  internal  layer  of 
hypo-eutectoid  steel.  The  main  points  to  be  noticed  are,  that 


5<C. 

1.4 

1.2 
1.0 
0.8 
0.6 
0.4 
0.2 

K 

\ 

\ 

\ 

\ 

\ 

v 

0       0.5        1       1.5        2       2.5        3  MM. 
FIG.  62. — Carburization  at  1830°  F.  for  Five  Hours  with  Ethylene.     (Giolitti.) 

the  case  contains  a  structure  with  greater  than  0.9  per  cent,  carbon, 
and  more  emphatically,  that  the  concentration  of  the  carbon  often 
diminishes  in  a  markedly  non-uniform  manner  or  discontinuity. 
Zones  of  this  type  are  always  found  in  carburizations  carried  out 
with  hydrocarbons;  they  also  are  typical  of  carburizations  obtained 
with  many  of  the  solid  carburizing  compounds  used  in  commercial 
work  in  which  the  action  of  the  hydrocarbons  greatly  predominates, 
or  in  the  presence  of  cyanides  (superficial  cementation). 

Of  the  specific  action  of  the  gaseous  hydrocarbons,  we  may  make 
the  following  remarks.  The  depth  or  velocity  of  penetration  in- 
creases, similarly  to  carbon  monoxide,  with  the  time  of  exposure. 
In  the  case  of  carburization  with  ethylene  and  methane,  the  cemented 
zones  obtained  in  a  definite  time,  although  likewise  increasing 


120  STEEL  AND  ITS  HEAT  TREATMENT 

markedly  in  thickness  with  rise  in  temperature,  other  things  remaining 
constant,  maintain  about  the  same  concentration  and  the  same  dis- 
tribution of  the  carbon  in  the  three  zones — thus  differing  widely 
from  carbon  monoxide.  In  contrast  with  the  use  of  these  pure 
gases,  the  use  of  hydrocarbons  in  practice  presents  a  different  aspect, 
especially  when  compared  with  the  use  of  carbon  monoxide  in  prac- 
tice. Contrary  to  the  simplicity  of  the  reactions  which  always 
characterize  the  cementation  by  carbon  monoxide,  the  complexity 
of  the  reactions  with  hydrocarbons  increases  enormously  in  industrial 
work.  The  gas  in  such  instances  does  not  consist  of  a  single,  chem- 
ically definite  hydrocarbon,  but  of  a  mixture  of  various  hydrocarbons. 
If  we  work  at  a  comparatively  low  temperature,  such  as  at,  or  slightly 


Hyper-eutectoid    ^  .  f 


Eutectoid 


Hypo-eutectoid 


FIG.  63. — Carburization  with  Hydro-Carbons.     X25.     (Bullens.) 

under,  the  upper  critical  range,  the  process  is  slow  and  non-uniform. 
At  the  high  temperatures  generally  used,  cemented  zones  of  exces- 
sively high  carbon  are  always  produced.  The  same  complexity  of 
reactions  make  it  difficult,  in  practice,  to  work  with  a  cement  having 
hydrocarbons  as  a  base,  either  as  a  mixture  of  solids  in  the  carburizing 
box,  or  as  gases  in  the  newer  processes,  in  such  a  way  as  to  obtain 
well-defined  results.  Thus  the  use  of  such  hydrocarbons  is  not 
advantageous  where  a  certain  value  of  maximum  concentration, 
combined  with  a  definite  distribution  of  that  carbon,  is  necessary 
in  carburized  steels  in  which  a  considerable  depth  is  desired. 

Enfoliation. — All  those  who  have  had  much  to  do  with  case 
hardening  and  its  products  are  familiar  with  the  flaking,  chipping,  or 
even  peeling  off  of  parts  of  the  case  from  the  remainder  of  the  steel. 


CASE  CARBURIZING 


121 


These  fractures  are  entirely  different  from  those  occurring  in  homo- 
geneous high-carbon  hardened  steels.  While  in  the  latter  the  frac- 
tures always  have  a  characteristic  conchoidal  form,  in  case-hardened 
steels  the  chipping  or  enfoliation  always  takes  place  along  a  line 
corresponding  to  the  separation  of  two  zones  exhibiting  markedly 
different  structure  or  "  grain."  A  microscopic  and  chemical  investi- 
gation brings  out  the  fact  that  this  line  or  plane  of  weakness  charac- 
terizes the  separation  of  the  hyper-eutectoid  zone  from  that  of  the 
hypo-eutectoid  zone,  or  at  a  carbon  content  corresponding  to  that  of 
about  0.90  per  cent.  Further,  this  plane  of  weakness  corresponds 
to  a  discontinuity  in  the  concentration  or  distribution  of  the  carbon 


1.4 
1.2 
1.0 
0.3 
0.6 
0.4 
0.2 


\ 


0.5        1       1.5        2      2.5        3  MM. 
FIG.  64.— €arburization  at  1920°  F.  for  Four  Hours  with  Ethylene.     (Giolitti.) 

which  is  characteristic  of  carburized  zones  of  the  hydrocarbon  type 
previously  described. 

It  is  now  evident  that,  in  order  to  eliminate  the  possibility  and 
dangers  of  this  enfoliation,  we  must 

(1)  obtain  a  gradual  and  progressive  change  in  the  distribution 
of  the  carbon  so  that  it  will  vary  from  the  minimum  of  the  core  to 
the  maximum  at  the  surface  of  the  case,  and  in  no  place  exhibit  the 
phenomenon  of  discontinuity;    . 

(2)  eliminate  the  possibility  of  discontinuity  at  the  eutectoid  by 
keeping  the  maximum  concentration  of  the  carbon  at  or  below  0.90 
per  cent,  carbon  (thus  eliminating  the  hyper-eutectoid  zone); 

(3)  and  in  any  case,  modify  by  suitable  heat  treatments  the 
structure  obtained  by  carburization. 


122  STEEL  AND  ITS  HEAT  TREATMENT 

Maximum  Carbon  Concentration. — Now  while  we  have  under 
(2)  advised  the  eutectoid  carbon  ratio  as  a  means  of  preventing 
enfoliation,  it  must  not  be  at  once  concluded  that  enfoliation  is  the 
direct  sequence  of  increasing  the  carbon  concentration  maximum 
to  over  0.9  per  cent.  Such  is  not  the  case  if  the  proper  heat  treatment 
methods  are  employed.  Unfortunately,  however,  the  majority  of 
commercial  plants  employing  case  hardening  do  not  either  under- 
stand, or  are  unable  to  put  into  practice,  the  methods  which  are 
necessary  when  the  carbon  content  of  the  case  runs  beyond  0.9  per 
cent,  carbon.  That  such  high  carbon  contents  are  undeniably 
advantageous  in  many  instances  where  it  has  been  generally  thought 
that  their  use  was  impossible  will  also  be  shown,  as  will  the  so-called 
"  secret  "  processes  of  treating  the  steel.  But  for  plants  which  are 
unable  to  employ  the  necessary  metallurgical  skill  and  appliances, 
it  will  be  far  better  to  adopt  such  case-hardening  processes  as  will 
turn  out  a  good  product  having  a  maximum  carbon  concentration  in 
the  case  of  about  0.9  per  cent.  The  further  advantages  of  this  will 
be  brought  out  under  the  discussion  of  heat-treatment  methods  in 
Chapter  VII. 

Intermediary  Type  of  Carburized  Zone. — Recognizing  under  these 
conditions  the  validity  of  not  exceeding  the  eutectoid  limit,  and  the 
obvious  advantages  of  preventing  discontinuity  between  the  core  and 
the  surface  of  the  case  regardless  of  the  maximum  carbon  content, 
it  is  evident  that  we  must  obtain  a  cemented  zone  intermediary 
between  those  of  the  two  general  types,  previously  described.  In 
other  words,  the  type  of  case  must  have  the  principal  character- 
istics of  the  carbon  monoxide  type,  but  which  are  modified — by 
increasing  the  carbon  content — by  cements  typical  of  the  hydro- 
carbons, or  other  suitable  procedure. 

Carbon  Monoxide  Plus  Hydrocarbons. — From  the  results  of 
experiments  carried  out  with  carbon  monoxide  plus  specific  amounts 
of  hydrocarbons,  Giolitti  shows  that  the  additive  effect  of  the  latter, 
as  compared  with  those  carried  out  with  pure  carbon  monoxide,  may 
be  summed  up  as  follows:  "  The  addition  of  small  quantities  of 
volatile  hydrocarbons  to  carbon  monoxide  merely  raises  the  con- 
centration of  the  carbon  in  the  external  layers  of  the  cemented  zones 
above  the  value  which  would  result  from  the  use  of  pure  carbon 
monoxide  under  identical  experimental  conditions.  This  increase 
is  greater  the  larger  the  proportion  of  the  hydrocarbon  contained  in 
the  gaseous  mixture,  as  long  as  this  proportion  does  not  reach 
a  value  such  that  the  velocity  with  which  the  free  carbon  is  formed 


CASE   CARBURIZIXG 


123 


by  the  decomposition  of  the  hydrocarbon  does  not  surpass  the  veloc- 
ity with  which  this  carbon  passes  through  the  stage  of  carbon  mon- 
oxide into  solution  in  the  iron.  From  this  limit  the  excess  of  carbon 
which  is  liberated  begins  to  deposit  on  the  steel  and  the  concentration 
of  the  carbon  in  the  external  layers  of  the  cemented  zone  reaches  the 
maximum  value  corresponding  to  that  which  is  obtained  by  cement- 
ing with  solid  cements,  or  with  cements  which  behave  as  such,  and 
from  this  point  on,  the  concentration  and  the  distribution  of  the 
carbon  in  the  cemented  zones  no  longer  vary  markedly,  even  if  the 
proportion  of  the  hydrocarbon  increases  greatly." 

"  From  what  precedes  it  is  evidently  possible  to  obtain,  by  means 
of  mixtures  of  carbon  monoxide  and  vapors  of  volatile  hydrocarbons, 
cemented  zones  in  which  the  maximum  concentration  of  the  carbon 


0.8 
0.6 
0.4 
0.2 


.2        .4 


.6 


1.2      1.4      1.6      1.8 


MM. 


FIG.  65. — Cemented  Zone,  Intermediate  Type,  Carburized  with  Carbon  Monox- 
ide Plus  3.1  per  cent.  Ethylene.     (Giolitti.) 

in  the  external  layers  has  a  definite  value,  lying  between  a  minimum 
corresponding  to  that  which  would  be  obtained  by  working  under 
the  given  conditions  with  pure  carbon  monoxide,  and  a  maximum 
which  would  be  obtained  by  working  with  vapors  of  the  hydrocarbon 
alone.  This  is  achieved  simply  by  using  gaseous  mixtures  containing 
a  proper  proportion  of  hydrocarbon  varying  with  the  conditions 
under  which  cementation  is  to  be  effected,  such  as  temperature, 
pressure,  relation  between  the  velocity  of  the  gaseous  current  and 
the  surface  of  the  steel  to  be  cemented,  etc." 

An  example  of  this  is  shown  by  the  concentration-depth  diagram 
in  Fig.  65,  the  results  of  which  were  obtained  experimentally  by 
cementing  0.26  per  cent,  carbon  steel  cylinders  for  four  hours  at  a 
temperature  of  1830°  F.  in  a  mixture  of  carbon  monoxide  with  3.1 
per  cent,  of  ethylene.  It  will  be  noted  that  the  carbon  decreases 
progressively  and  in  a  slow  and  uniform  manner,  but  that  the  addi- 


124  STEEL  AND  ITS  HEAT  TREATMENT 

tion  of  the  hydrocarbon  has  raised  the  maximum  carbon  content  up 
to  nearly  the  eutectoid  ratio.  Thus,  in  this  case,  there  has  been  pro- 
duced a  carburized  zone  of  an  intermediary  type  which  fulfils  the 
requirements  stated  for  the  avoidance  of  enfoliation. 

Following  along  these  lines  of  using  a  gaseous  mixture  consisting 
of  certain  proportions  of  carbon  monoxide  gas  and  the  volatile 
hydrocarbons,  several  industrial  methods  have  been  worked  out,  and 
which  have  given  excellent  satisfaction.  The  application  of  the 
same  theory  is  also  applicable  to  the  commercial  solid  cements  in 
which  the  necessary  gases  are  evolved  during  the  heating  operation, 
but  on  account  of  the  greater  lack  of  control  the  variations  to  be 
obtained  are  necessarily  of  considerable  extent. 

Carbon  Plus  Carbon  Monoxide. — As  we  have  stated,  the  carburiz- 
ing  action  of  solid  carbon  in  the  absence  of  all  gases  is  commercially 
negligible.  But  by  introducing  oxygen  which  will  form  the  gaseous 
vehicle  (carbon  monoxide),  or  by  adding  carbon  monoxide  directly, 
the  presence  of  solid  carbon  greatly  intensifies  the  carburization. 
Thus,  similarly  to  definite  mixtures  of  carbon  monoxide  plus  hydro- 
carbons, the  desirable  form  of  the  intermediary  type  of  carburized 
zone  may  be  obtained  by  carbon  monoxide  in  the  presence  of  solid 
cements. 

By  varying  the  various  factors  of  temperature,  time  of  exposure, 
pressure  of  gas,  etc.,  the  use  of  a  mixed  cement  may  be  varied  within 
wide  limits,  and  with  the  production  of  a  hyper-eutectic  zone  if  so 
desired.  This  latter  comes  into  great  practical  use  when  it  is  desired 
to  produce  zones  of  considerable  width.  Co-ordinated  with  this  is 
the  use  of  carbon  monoxide  as  an  "  equalizer,"  that  is,  by  first  carry- 
ing out  the  carburization  process  in  the  usual  way  (with  mixed 
cements),  the  maximum  concentration  of  the  carbon  may  be  made 
quite  high;  this  is  then  followed  by  the  use  of  carbon  monoxide  alone 
(without  the  presence  of  granular  carbon).  By  these  means  the 
concentration  of  the  carbon  may  be  lowered — by  the  distributive 
action  of  the  carbon  monoxide,  to  such  maximum  concentration  as 
may  be  desired.  This  is  graphically  shown  in  Fig.  66,  by  Giolitti. 
The  steel  used  was  of  the  composition : 

Per  cent. 

Carbon 0.12 

Manganese 0 . 47 

Phosphorus..  .  . . 0.03 

Sulphur 0.02 

Silicon..  ..0.06 


CASE  CARBURIZING 


125 


Curve  a  shows  the  concentration  depth  after  carburization  for  ten 
hours  at  2010°  F.  with  mixed  cement.  Curve  b  represents  the 
results  after  heating  the  preceding  for  five  hours  at  the  same  tem- 
perature in  "  isolated  "  carbon  monoxide.  Curve  c  gives  the  results 
after  another  five  hours  heating  at  the  same  temperature  in  "isolated" 
carbon  monoxide.  Thus  we  see  that  the  curves  have  undergone 
a  gradual  change  in  form  and  position  due  to  the  action  of  carbon 
monoxide  alone.  Such  methods  as  these  will  permit  of  the  elimina- 
tion of  the  dangerous  hyper-eutectoid  zone,  and  at  the  same  time  give 

jto. 

1.3- 

1.2- 

1.1 

1.0 

0.9- 

0.8- 

0.7 

0.6- 

0.5- 

0.4- 

0.3- 

0.2 

o.i  H 


9  MM. 


FIG.  66. — Distributive  Action  of  Carbon  Monoxide.     (Giolitti.) 

all  the  benefits  to  be  obtained  from  a  carburized  zone  of  the  inter- 
mediate type  previously  described. 


TEMPERATURE    AND    TIME    FACTORS 

Solution  of  the  Carbon. — We  have  seen  how  certain  gases,  by 
diffusing  into  the  steel,  precipitate  free  carbon  within  the  steel. 
Now  this  carbon,  under  suitable  conditions,  may  be  dissolved  at  once 
by  the  iron,  forming  a  true  steel.  It  is  evident  that  the  solubility  of 
this  carbon  (or  carbide)  must  depend  upon  the  allotropic  condition  of 
the  iron,  which,  in  turn,  will  depend  upon  the  temperature.  As  we 
have  explained  in  previous  chapters,  iron  may  be  held  in  the  alpha, 
beta  or  gamma  state.  Thus,  if  a  piece  of  normal  0.2  per  cent,  carbon 


126  STEEL  AND  ITS  HEAT  TREATMENT 

steel  is  heated,  none  of  the  cementite  which  is  mechanically  mixed 
with  ferrite  (iron)  to  make  up  the  mechanical  mixture  pearlite  is 
affected  until  the  lower  critical  temperature  of  about  1350°  F.  is 
reached.  At  this  temperature  the  iron  of  the  pearlite,  previously 
in  the  alpha  condition,  changes  into  gamma  iron  and  dissolves  the 
cementite,  the  two  forming  a  solid  solution  or  austenite.  In  other 
words,  it  is  necessary  for  the  iron  to  be  in  a  higher  allotropic  condition 
than  that  of  the  alpha  stage  in  order  to  dissolve  carbon  (or  carbide) . 
As  the  temperature  of  the  steel  is  progressively  raised,  more  and  more 
of  the  excess  iron  is  dissolved  by  the  austenite,  until  at  the  Ac3  range 
the  whole  mass  of  the  steel  consists  of  austenite  and  has  all  the 
iron  in  the  gamma  state. 

Thus  we  see  that  while  it  is  possible  for  carburization  to  take 
place  at  temperatures  varying  between  the  Acl  and  Ac3  ranges, 
the  carburization  must  necessarily  be  not  only  slow,  but  also  irreg- 
ular and  non-uniform.  In  other  words,  the  minimum  temperature 
which  should  be  used  in  industrial  carburization  should  not  be 
lower  than  the  upper  critical  range  of  the  initial  steel  to  be 
carburized. 

Depth  of  Penetration. — All  commercial  carburizing  processes 
must  provide  a  depth  of  case  which  will  satisfy  the  requirements  of 
the  specific  use  to  which  the  steel  will  be  put  in  practice.  Thus 
many  parts  will  only  require  a  depth  of  case  of  say  -^  or  ^  of  an 
inch,  or  if  grinding  is  necessary,  this  may  be  increased  to  y^  of  an 
inch;  other  parts,  such  as  armor  plate,  may  require  a  considerable 
thickness.  With  a  definite  depth  of  case  in  view,  economic  consider- 
ations require  that  the  velocity  of  penetration  shall  be  definitely 
known  in  relation  to  the  factors  of  time  and  temperature,  the  nature 
of  the  carburizing  agent,  and — in  the  case  of  gases — such  other  factors 
as  pressure,  quantity  of  gas,  etc. 

The  penetration  of  carbon  (differentiating  this  depth  of  penetra- 
tion from  the  distribution  of  the  carbon)  increases  with  the  temper- 
ature and  with  the  time  of  exposure,  but  not  always  in  direct  pro- 
portion to  these  two  factors.  Given  a  definite  temperature  and 
carburizing  compound,  it  may  be  said  in  general  that  the  carburiza- 
tion commences  and  continues  at  a  comparatively  high  rate  of  speed 
until  the  outer  layers  are  saturated  with  carbon — dependent,  of 
course,  upon  the  nature  of  the  cement;  there  is  then  a  drop  in  the 
rate  of  carburization,  varying  according  to  the  temperature,  and  this 
in  turn  is  followed  by  a  velocity  of  penetration  which  seems  to  be 
more  nearly  proportional  to  the  length  of  exposure. 


CASE  CARBURIZING 


127 


These  facts  are  shown  graphically  in  Figs.  67  and  68,  the  former 
illustrating  this  velocity  of  penetration  particularly  for  short  expos- 
ures, while  the  latter  emphasizes  the  effect  of  long  continued  heating. 
In  each  case  the  experiments  were  carried  out  under  conditions  usu- 
ally adopted  in  industrial  establishments.  The  bars  of  soft  steel 
were  allowed  to  soak  at  the  stated  temperatures  for  definite  lengths 
of  time  and  the  penetration  was  then  measured.  The  results  from 
which  these  graphs  were  plotted  were  reported,  in  the  first  instance, 


1234567 

FIG.  67. — Velocity  of  Penetration,  Short  Exposures. 

• 

by  a  company  using  a  cement  of  the  barium  carbonate-carbon- 
aceous type,  and  in  the  second  case  by  Giolitti,  using  a  common 
commercial  cement  consisting  of  ground-wood  charcoal  treated 
with  5  per  cent,  potassium  ferrocyanide  and  mixed  with  an  equal 
weight  of  barium  carbonate. 

In  the  case  of  the  first  compound  it  is  interesting  to  note  that 
there  is  a  decided  decrease  in  the  relative  rate  of  penetration  when 
the  depth  of  case  reaches  approximately  0.05  in.  This  cycle 
appears  to  take  place  in  the  same  order  at  all  temperatures  used, 
with  the  difference  that  the  relative  speeds  of  penetration  are  higher 


128 


STEEL  AND   ITS  HEAT  TREATMENT 


at  higher  temperatures,  although  not  proportionally  so.  Thus  we 
may  coin  a  phrase  and  call  this  depth  of  0.05  in.  the  "  critical 
penetration"  of  this  individual  compound.  Considering  the  depth 
of  penetration  only,  and  disregarding  other  economic  and  technical 
factors  which  may  enter  into  consideration,  it  is  evident  that  this 
particular  compound  may  be  used  to  good  advantage  when  a  case 
corresponding  to  about  0.05  in.  is  desired.  To  this  depth  the 
steel  will  be  carburized  at  a  maximum  speed  or  velocity,  and  there- 


.14 


.12 


.10 


.08 


f 


12        24        36        48        60        72        81         %       103      120  Hours 
FIG.  68. — Velocity  of  Penetration,  Long  Exposures.     (Giolitti.) 


fore  at  the  minimum  furnace  or  heating  cost.  With  this  particular 
compound  simply  as  an  illustration,  any  other  commercial  carburizing 
compound  might  be  studied  from  the  practical  side  for  the  determina- 
tion of  this  critical  depth  of  penetration,  and  use  made  of  the  results 
to  good  advantage  in  reducing  operation  costs. 

The  influence  of  temperatures  of  carburization  under  the  upper 
critical  range  upon  the  depth  of  penetration  and  the  maximum  car- 
bon content  is  also  brought  out  by  a  comparison  of  Figs.  69  and  70 
These  photomicrographs  represent  the  effect  of  cementation  upon  a 
0.11  per  cent,  carbon  steel  carburized  for  one  hour  in  a  mixture  of 


CASE  CARBURIZING 


129 


charcoal  and  barium  carbonate,  but  at  different  temperatures. 
Fig.  69  shows  the  results  of  a  temperature  under  the  A3  range; 
Fig.  70  the  cementation  at  a  temperature  considerably  over  the  A3 
range.  The  practical  value  is  self-evident. 

Liquation. — Sudden  variations  in  the  concentration  of  the  carbon 
in  the  cemented  zone  may  be  manifested  when  intense  carburiza- 
tion  is  effected  at  high  temperatures,  and  the  carburized  pieces  are 
allowed  to  cool  slowly  through  a  more  or  less  wide  interval  of  tem- 


,  3~'"A-  £ •  . -^*"  7  -   ..  '•*&*.'•£?  -    -  Vv 


're'vZ^~  »-  '^'  *  '''^^w^-i;-^ 

./>.'  ...dr.  ..«"-••*.,'-.  •?-  *      V«."^"iV.  »  >ty 


FIG.  69. — Carburization  of  0.11  per 
cent.  Carbon  Steel  at  a  Temper- 
ature under  the  Upper  Critical 
Range  with  BaCO3  and  Char- 
coal, for  One  Hour.  (Nolly 
and  Veyret.) 


•>'  **  *  '!A**  •  V  *  '    ^  ••"*•  V  •*  •V*'^ 

!    K"*\  >  *  "  '^  f     '••C^i^ 

^  *"      *  »    V;    "''  »^*SJy*'4^*^'"''»^^f 


FIG.  70. — Carburization  of  0.11  per 
cent. Carbon  Steel  at  a  Temper- 
ature Considerably  Over  the 
Upper  Critical  Range  with 
BaCOa  and  Charcoal,  for  One 
Hour.  (Nolly  and  Veyret.) 


perature  before  being  quenched.  This  variation  consists  of  a  true 
liquation  of  the  cement ite  (and  of  the  ferrite)  during  their  segrega- 
tion front  the  solid  solution.  Take,  for  example,  the  diagrams  in 
Figs.  71  and  72,  representing  the  results  obtained  by  carburizing  a 
0.26  per  cent,  carbon  steel  for  four  hours  at  1830°  F.  in  ethylene,  with 
the  difference,  however,  that  the  carburized  steel  represented  by 
Fig.  71  was  cooled — during  32  minutes — to  a  temperature  of  1380°  F. 
and  then  quenched,  while  that  of  Fig.  72  was  quenched  immediately 
following  Carburization  from  that  temperature  (1830°). 

Comparing  the  two  diagrams  we  see  that,  while  the  concentration 
of  the  carbon  in  Fig.  72  decreases  continuously  and  uniformly  as  we 


130 


STEEL  AND   ITS  HEAT  TREATMENT 


proceed  from  the  surface  towards  the  core,  that  in  Fig.  71  shows  a 
marked  increase,  followed  by  a  very  rapid  decrease,  before  it  exhibits 


*c. 

1.6 

1.4 
1.2 
1.0 
0.8 
0.6 
0.4 
0.2 

/ 

/ 

\ 

^ 

^ 

\ 

^ 

-—  _,___ 

""""- 

\ 

•*N^^ 

\. 

^_ 

1.4      1.0       1.8 


2.2  MM. 


FIG.  71. — Liquation  of  Cementite  through  Slow  Cooling.     (Giolitti.) 

that  gradual  decrease  which  characterized  the  first  case.     As  the 
carburization  proper  was  identical  in  both  cases,  it  is  evident  that 


JfC. 
JL6 


±2 

1.0 
0.8 
0.6 
<U 
03 


.*        .4        .6        .8         1       1.2      1.4      1.6      1.8     MM. 

FIG.  72. — Prevention  of  Liquation  by  Quenching,     (Giolitti.) 

the  discontinuity  in  the  carbon  distribution  must  be  due  to  the  dif- 
ference in  the  rate  of  cooling  following  the  carburization  proper. 


CASE  CARBURIZING 


131 


Thus  we  see  that  the  liquation,  or  accumulation,  of  the  cementite 
in  the  external  layers  will  tend  to  emphasize  the  line  of  demarkation 
between  the  hyper-  and  hypo-eutectoid  zones.  And  this,  in  turn,  will 
magnify  the  dangers  of  enfoliation.  Fig.  73  is  a  photomicrograph 
from  a  piece  of  carburized  steel  which  failed  in  service;  the  cause 
of  enfoliation  in  this  case  is  undoubtedly  due  to  this  phenomenon 
of  liquation. 

This  is  another  reason  why  those  processes  of  carburization  should 
be  used  which  will  avoid  the  formation  of  the  hyper-eutectoid  zone, 


x*. 

'^j^a&£''^P 

^^ati^** 

FIG.  73. — Failure  Due  to  Liquation.     (Giolitti.) 

and  thus  eliminate  the  possibility  of  enfoliation  through  differences 
in  carbon  distribution  caused  by  accumulation  of  the  free  cementite. 
However,  if  cements  which  give  carburized  zones  of  the  hydro- 
carbon type  are  used,  it  is  apparent  that  this  accumulation  may  be 
avoided  by  quenching  from  the  temperature  of  carburization. 
Although  this  is  possible  in  the  newer  processes  of  using  either  a 
gaseous  mixture  or  a  mixed  cement,  it  is  manifestly  impossible  under 
the  older  methods  of  using  solid  cements.  We  will  shortly  describe 
methods  of  heat  treatment  by  which  the  effect  of  the  cementite 
accumulation  may  be  corrected  in  the  majority  of  instances,  as  well 


132  STEEL  AND   ITS  HEAT  TREATMENT 

as  giving  reasons  why  quenching  from  the  temperature  of  carburiza- 
tion — when  it  is  1750°  F.  or  more — is  technically  bad. 

Oscillating  Temperatures. — There  is  still  another  effect  of  tem- 
perature which  should  be  mentioned  in  this  connection  on  account 
of  its  action  in  many  industrial  carburizing  processes.  This  is  the 
effect  of  non-uniform  or  oscillating  temperatures  during  carburiza- 
tion,  so  often  met  with  in  practice.  Without  going  into  the  theo- 
retical explanations  involved,  it  has  been  shown  by  Giolitti  and 
Scavia  that,  where  under  normal  conditions  the  formation  of  free 
cementite  cannot  take  place,  by  carrying  out  the  carburization  under 
identical  conditions  but  with  variable  temperatures  oscillating  within 
definite  intervals,  the  occurrence  of  free  cementite  will  result.  The 
industrial  importance  of  this  is  evident.  In  the  first  place  it  explains 
the  abnormal  increase  in  free  cementite  which  is  sometimes  met  with 
in  practice  in  which  the  normal  nature  of  the  carburization  should 
produce  cemented  zones  of  the  intermediate  type  and  the  absence  of 
free  cementite,  besides  demonstrating  the  necessity  of  maintaining 
uniform  temperatures  throughout  the  whole  carburization  within 
a  very  close  range.  And  in  the  second  place  it  furnishes  a  means  of 
carrying  out  the  heating  during  carburization  in  such  a  way  as 
purposely  to  cause,  with  certainty,  the  formation  of  free  cementite 
when  it  is  desired  to  obtain  cemented  zones  capable  of  taking  an 
exceedingly  high  degree  of  hardness  by  quenching  without  its  being 
necessary  that  their  brittleness  be  reduced  to  a  minimum. 

Temperature  Factors. — Although  much  has  been  said  and 
written  concerning  the  effect  of  temperature  upon  carburization,  the 
majority  have  confused  the  velocity  or  depth  of  the  carbon  concen- 
tration with  the  maximum  concentration  and  distribution  of  the  car- 
bon. While  it  has  been  shown  that  the  depth  of  penetration  is,  in 
general,  a  direct  function  of  the  temperature  with  all  commercial 
carburizing  mixtures,  the  same  is  not  entirely  true  of  the  maximum 
concentration  of  the  carbon.  In  fact,  through  the  study  of  pure 
gases  such  as  we  have  indicated,  it  has  been  shown  that  in  the  case 
of  carbon  monoxide  alone  the  maximum  concentration  of  the  carbon 
actually  decreases,  other  things  being  equal,  with  increase  in  the 
temperature  of  carburization. 

In  the  cases  of  the  newer  processes  involving  the  uses  of  gases  or 
a  mixed  cement,  the  effect  of  such  temperature  on  the  maximum 
concentration  and  distribution  of  the  carbon  may  be  practically 
varied  at  will  by  a  change  in  the  other  factors  previously  mentioned. 
In  other  words,  on  account  of  the  almost  absolute  control  with  which 


CASE  CARBURIZING  133 

such  processes  of  carburization  may  be  regulated,  the  actual  effect  of 
the  temperature  is  minimized  and  does  not  play  the  important  part 
which  is  manifested  in  the  older  processes  involving  the  use  of  solids 
alone. 

Influence  of  Temperature  on  Different  Cements. — The  com- 
plexity and  lack  of  control  of  the  reactions  involved  in  the  use  of 
solid  cements  make  the  factor  of  temperature  extremely  important. 
Thus  some  cements,  such  as  charcoal  plus  barium  carbonate,  which 
at  the  lower  temperatures  give  "  gradual  "  or  "  mild  "  cases,  at  the 
higher  temperatures  may  act  as  "  sudden  "  or  "  quick  "  cements. 
A  consideration  of  the  common  solid  cements  (of  which  we  are  now 
speaking)  as  used  in  general  commercial  work  would  tend  towards 
the  conclusion  that  their  action,  on  the  whole,  is  more  gradual  at 
the  lower  temperatures  than  at  the  higher  temperatures.  Further, 
it  might  even  be  said  that,  other  things  being  equal,  the  higher  the 
temperature  of  carburization  the  higher  will  be  the  maximum  carbon 
concentration  in  the  case.  Although  these  statements  may  not  hold 
good  for  all  instances  in  which  carbon  is  used  as  the  base,  their  prac- 
tical working  out  is  generally  evidenced  in  the  general  average  of 
thin  cemented  zones  found  in  commercial  case  hardening. 

Low  Temperature  Carburization. — Now  as  the  majority  of  case- 
hardened  products  require  the  elimination  of  brittleness,  both  in 
case  and  core,  and  as  the  use  of  "  gradual  "  cements  at  the  lower 
temperatures  of  carburization  will  advance  that  condition  of  affairs 
(i.e.,  the  formation  of  cemented  zones  of  the  intermediate  type, 
showing  the  absence  of  the  hyper-eutectoid-zone,  and  a  gradual 
distribution  of  the  carbon  concentration  from  the  external  surface 
of  the  case  to  the  core)  the  use  of  the  lower  temperatures  of  carbur- 
ization is  coming  more  and  more  into  vogue.  That  is,  the  tendency 
is  to  use  moderate  heats  and  maintain  them  for  a  length  of  time 
sufficient  to  obtain  a  reasonable  depth  of  case.  These  heats  may  be 
said,  in  a  general  way,  to  correspond  with  temperatures  of  about 
100°  F.  over  the  upper  critical  range  of  the  steel  to  be  carburized. 
Although  the  depth  of  case  is  largely  dependent  upon  the  temper- 
ature, as  well  as  upon  the  time  of  carburization,  under  the  above  con- 
ditions it  should  be  considered  poor  practice  to  raise  the  temperature 
to  the  high  limit  simply  for  the  purpose  of  reducing  the  time  element; 
the  repair  items  on  the  furnace  will  increase,  the  fuel  cost  will  be 
greater,  and— above  all — the  maximum  carbon  concentration  and 
the  relation  of  the  various  zones  to  each  other  so  changed  that 
the  whole  character  of  the  finished  product  may  be  altered. 


134 


STEP:L  AND  ITS  HEAT  TREATMENT 


Relation  of  Temperature  to  Grain-Size. — Another  important 
feature  in  determining  whether  or  not  to  use  a  high  temperature 
for  case  carburizing  is  the  relation  of  such  temperature  to  the  grain 
size.  As  has  been  explained  in  previous  chapters,  upon  passing  the 
Acl  range  on  heating  the  grain  size  begins  to  coarsen,  and  most 
noticeably  so  after  passing  the  upper  critical  range  or  the  low  carbur- 
izing temperatures.  Thus  from  about  1550°  F.  and  so  on  up  to 
1850°  or  1900°-  F.  (which  are  about  the  maximum  temperatures 
used)  the  grain  increases  in  size  most  markedly.  This  increase 
in  grain  size  has  a  direct  bearing  upon  the  impact  strength  of  the 
steel,  as  is  shown  by  the  following  results  (by  Guillet)  obtained  by 
annealing  case-hardening  steel  bars  at  the  temperatures  given  for 
eight  hours,  and  which  under  the  conditions  of  a  normal  annealing 
gave  an  impact  test  of  28  kilogram-meters. 


Temperature, 

Impact  Test, 
Kilogram-metres. 

1470 

26 

1560 

28 

1650 

15 

1740 

12 

1830 

4 

1920 

3 

2010 

4 

Thus  it  will  be  seen  that  any  heating  of  long  duration  at  temperatures 
above  the  upper  critical  range  (the  low  case  carburizing  temperature) 
greatly  lowers  the  resistance  of  the  steel  to  impact  or  shock.  When 
high  temperature  carburizing  is  necessary,  however,  suitable  treat- 
ment after  carburizing  may  be  used  to  "  regenerate  "  the  core. 
Even  then,  however,  the  effect  of  long-continued  heating  at  high 
temperatures  is  often  manifest. 

High-temperature  Carburization. — Although  the  advisability 
of  using  the  lower  temperatures  for  case  carburizing  has  been 
emphasized,  it  must  not  be  thought  that  the  higher  temperatures 
of  1800°  F.,  or  even  higher,  are  never  to  be  used.  Contrarily,  the 
latter  are  often  mandatory  in  certain  classes  of  work  where  speed 
of  penetration  is  the  first  requisite,  or  where  low  cost  of  production 
is  necessitated  and  the  absence  of  brittleness  is  not  a  prime  factor. 
If  overheating  and  burning  are  suitably  guarded  against,  and  the 
methods  of  packing  are  such  as  will  keep  warping  and  distortion  at  a 
minimum,  and  the  carburizing  process  is  followed  by  a  technically 


CASE  CARBURIZING  135 

adjusted  series  of  heat  treatment  operations,  most  excellent  results 
can  be  obtained.  In  fact,  it  is  only  within  comparatively  recent 
years  that  the  use  of  the  lower  temperatures  has  been  practiced 
to  any  great  extent;  a  decade  or  so  ago  if  1550°  F.  or  thereabouts 
had  been  suggested  as  giving  greater  efficiency,  the  proposal  would 
have  been  laughed  at  by  the  majority  of  "  practical  "  hardeners. 

COMMERCIAL  DATA 

Simple  Solid  Cements. — In  the  foregoing  sections  we  have 
attempted  to  give  in  brief  the  underlying  principles  which  govern  all 
processes  of  partial  carburization.  By  the  use  of  these  principles 
we  have  further  shown  that,  in  carburizations  with  gaseous  or  certain 
mixed  cements,  it  is  possible  to  so  select  the  conditions  of  carburiza- 
tion as  to  obtain  with  certainty  cemented  zones  of  predetermined 
and  definite  form.  Similarly,  the  same  principles  are  applicable 
to  carburization  with  solid  cements,  although  (as  we  have  shown) 
it  is  not  possible  to  even  approximate  the  same  accuracy  on  account 
of  the  lack  of  control  of  such  cementations.  Nevertheless,  a  study  of 
the  preceding  pages  should  convince  one  of  the  importance  of  using 
carburizing  compounds,  the  composition  and  manner  of  acting  of 
which  are  definitely  known.  With  such  principles  and  ideas  in  mind, 
it  should  be  comparatively  easy  for  each  one  to  prepare  for  himself  those 
cements  which  are  much  more  simple,  effective,  and  of  less  cost  than 
many  of  those  purchased  from  dealers  at  high  prices,  under  fancy  names, 
and  of  unknown  composition.  With  these  thoughts  in  mind,  we  will 
briefly  discuss  some  of  the  more  simple  compounds  used  in  commercial 
work. 

Wood  Charcoal. — Finely  divided  carbon  is  the  simplest  of  the 
solid  cements,  the  purest  form  in  commercial  practice  being  wood 
charcoal.  As  we  have  seen,  the  carburizing  activity  of  powdered 
wood  charcoal  is  dependent  upon  the  formation  and  action  of  carbon 
monoxide,  and  which  is  further  diluted  by  the  nitrogen  of  the  occluded 
air.  It  is  *  evident  that  the  use  of  this  charcoal  in  the  ordinary 
short  carburizations  for  the  production  of  cases  of  ^j  to  ^  in.  in 
thickness  will  have  the  tendency  to  give  cemented  zones  of  low 
and  irregular  carbon  content.  Its  use  for  the  deeper  carburizations, 
however,  may  be  distinctly  advantageous,  as  it  opposes  the  formation 
of  zones  too  high  in  carbon.  Figs.  74  and  75  show  the  effect  of  tem- 
perature upon  carburizations  with  wood  charcoal,  the  first  figure 
representing  carburizing  at  1560°  F.,  and  the  second  photo- 
micrograph at  1925°  F, 


136 


STEEL  AND  ITS  HEAT  TREATMENT 


Animal  Charcoal. — Thus,  for  thin  cases,  wood  charcoal  is  gen- 
erally mixed  with  certain  proportions  of  the  less  pure  charcoals, 
such  as  those  produced  by  the  carbonization  or  charring  of  leather, 
bones,  hoofs,  horns,  hair  and  other  animal  refuse,  etc.  In  these 
last  cements  we  therefore  have  the  action  of  pure  carbon  or  char- 
coal greatly  intensified  through  the  generation  of  volatile  hydro- 
carbons. We  will  later  mention  the  influence  of  the  phosphorus 
and  sulphur  which  these  cements  may  contain.  By  mixing  wood 


^p^fs^^p 


-  .v.-.-^v--*.  i*?^jap-vT4' 

«-:''^i^-^.^»t 

"•  r  /  .->*&««•  -••  -  -  &/•.: t-"^:  *^ 


FIG.  74. — Carburization  with  Char- 
coal for  One  Hour  at  1560°F., 
0.11  per  cent.  Carbon  Steel. 
(Nolly  and  Veyret.) 


?-i/vv: 


y.vt«j":»  rr..,,  -    -v. 

^'Kisi 

'^'tit/rfv:^^^^^ 


FIG.  75. — Carburization  with  Char- 
coal for  One  Hour  at  1925°  F., 
0.11  per  cent.  Carbon  Steel. 
(Nolly  and  Veyret.) 


charcoal  with  definite  proportions  of  these  animal  charcoals,  the 
carburizing  action  may  be  roughly  adjusted  between  the  minimum 
value  to  be  obtained  with  wood  charcoal  and  the  maximum  value 
of  the  animal  charcoals.  Of  the  more  common  "  mild  "  cements 
thus  obtained  we  may  mention  the  following: 

Parts 

a.  Powdered  oak  charcoal 5 

Powdered  leather  charcoal 2 

Lampblack 3 

b.  Wood  charcoal 7 

Animal  charcoal .3 


CASE  CARBURIZING  137 

Parte 

c.  Powdered  beech  charcoal 3 

Powdered  horn  charcoal 2 

Powdered  animal  charcoal 2 

Common  Salt. — Common  salt  (sodium  chloride)  is  used  in  many 
works  in  addition  to  charcoal,  it  seeming  to  give  better  results  than 
wood  charcoal  alone.  Exactly  what  is  its  specific  action  is  not 
thoroughly  understood.  Thus  we  have  the  mixture: 

Parts 

Wood  charcoal 7  to  9 

Common  salt 3  to  1 

Barium  Carbonate. — One  of  the  best  solid  cements  for  general  use 
is  that  consisting  of: 

Parts 

Barium  carbonate .  . 40 

Powdered  wood  charcoal 60 

Its  action  is  well  known  and  is  as  we  have  previously  described. 
For  cases  of  small  depths  it  gives  carburized  zones  markedly  more 
homogeneous  than  those  furnished  by  other  solid  cements.  Giolitti 
sums  up  its  advantages  as  follows:  "  In  general,  the  maximum  con- 
centration of  the  carbon  in  the  cemented  zones  obtained  with  carbon 
and  barium  carbonate  at  temperatures  between  1650°  and  2010°  F. 
varies  from  a  minimum  of  about  0.7  per  cent.,  for  the  very  thin  zones 
obtained  near  1650°,  to  a  maximum  of  about  1.3  per  cent,  for  the 
zones  thicker  than  1  mm.  (0.04  in.)  obtained  near  2010°  F. 

"  Another  advantage  of  this  cement  lies  in  its  property  of  being 
'  regenerated  '  easily  and  spontaneously  when  it  is  left  exposed  in  a 
thin  layer  to  the  air,  after  having  been  used  in  the  usual  manner. 
This  process  of  regeneration  is  due  to  the  fact  that  the  barium  oxide 
formed  during  cementation  by  the  dissociation  of  the  barium  car- 
bonate, absorbs  carbon  dioxide  from  the  air,  again  forming  barium 
carbonate.  After  a  certain  number  of  alternating  cementations  and 
regenerations  it  is  necessary  to  add  some  wood  charcoal  to  the  cement 
to  replace  that  burned  during  the  cementation  and  during  the  dis- 
charging of  the  boxes. 

"  The  preparation  of  this  cement  consists  simply  in  finely  grind- 
ing and  intimately  mixing  the  wood  charcoal  and  barium  carbonate. 
If  the  natural  barium  carbonate  (witherite)  is  used,  it  is  necessary 
to  powder  it  carefully  before  adding  it  to  the  carbon;  the  finely 


138 


STEEL  AND  ITS  HEAT  TREATMENT 


divided  precipitated  barium  carbonate,  on  the  contrary,  can  be 
mixed  directly  with  the  granulated  carbon  and  the  one  operation  of 
grinding  the  carbon  can  be  used  for  preparing  the  mixture." 

It  is,  of  course,  not  always  necessary  to  use  the  above  mixture 
ratio  of  40-60,  although  this  combination  has  been  shown  to  give 
about  as  good  results  as  may  be  obtained.  The  conditions  of  heat- 
ing, temperature,  size  of  the  pieces,  type  of  carburization  box  and 
method  of  packing,  etc.,  will  alter  each  individual  carburization, 
and  experiments  should  be  made  to  determine  as  exactly  as  possible 
the  proper  combination  of  the  different  factors  of  carburization  which 


14 


16 


24  6  8  10 

Duration  of  the  Heating  (Hours) 

FIG.  76. — Carburization  with  Common  Carburizing  Compounds. 


(Scott.) 


enter  into  consideration.  One  of  the  governing  factors  which  is  often 
overlooked  is  the  action  of  the  charcoal,  dependent  upon  its  composi- 
tion. Thus  much  of  the  ordinary  commercial  charcoal  still  contains 
considerable  volatile  or  organic  matter  (hydrocarbons)  which  may 
distinctly  alter  the  effect  of  the  carburizing.  In  order  to  reduce 
the  intensifying  action  of  such  constituents,  and  to  reduce  the  forma- 
tion of  the  hyper-eutectoid  zone,  it  is  always  advisable  first  to 
calcine  the  charcoal  before  using. 

The  general  relation  existing  between  the  depth  of  penetration 
due  to  charcoal,  charred  leather,  and  the  usual  40-60  barium  car- 
bonate-charcoal mixture  is  graphically  shown  in  Fig.  76,  obtained  by 
Scott  in  the  carburization  of  soft  steel  bars  at  1650°  F. 


CASE  CARBURIZING  139 

Gradual  Cements. — The  cements  which  we  have  just  enumerated 
are  generally  classed  as  "  gradual,"  for  reasons  previously  given. 
Yet  on  the  other  hand,  these  same  cements,  under  different  con- 
ditions of  carrying  out  the  carburization,  act  as  "  sudden  "  or 
"  quick  "  cements.  Thus  the  barium  carbonate  mixture  when  used 
at  low  temperatures,  or  for  the  carburization  of  pieces  of  large 
dimension  which  heat  up  slowly,  may  furnish  cemented  zones  in 
which  the  maximum  carbon  concentration  may  not  be  over  the 
eutectoid  ratio  (0.9  per  cent.).  The  same  mixture,  on  the  con- 
trary, may  become  a  sudden  cement  at  the  very  high  temperatures 
and  in  carburizing  objects  of  small  dimensions. 

Other  Solid  Cements. — In  addition  to  the  use  of  charcoal  plus 
the  animal  charcoals,  barium  carbonate  and  common  salt,  the  other 
agents  which  may  be  added  are  innumerable.  To  give  a  list  of  them 
would  occupy  several  pages,  besides  leading  to  the  inevitable  con- 
clusion that  the  efficacy  of  the  majority  of  them  is  small,  or  might 
be  even  detrimental.  For  it  should  be  stated  with  emphasis  that 
the  more  simply  and  more  chemically  definite  a  cement  can  be  made, 
the  greater  will  be  the  industrial  advantages. 

Sudden  Cements. — The  nature  of  the  most  important  of  these 
additions  is  to  make  the  mixture  act  quickly,  giving  rise  to  a  thin 
cemented  zone  of  high  carbon  in  a  very  short  interval.  Thus  we 
have  the  use  of  coke  saturated  with  mineral  oil,  of  the  saturation  of 
the  charcoal  in  solutions  of  cyanides  or  ferrocyanides,  and  of  the 
presence  in  greater  or  less  quantities  of  the  concentrated  salts  of 
cyanogen  as  specific  additions.  Of  those  used  in  practice,  the  follow- 
ing example  is  extremely  interesting; 

11  Ibs.  prussiate  of  potash, 
30  Ibs.  sal  soda, 
20  Ibs.  coarse  salt, 

6 .bushels  powdered  hickory  charcoal, 
30  quarts  water. 

Grenet  recommends  the  following  cements  which  have  given  good 
results  in  practice: 

Parts. 

a.  Powdered  wood  charcoal 1 

Salt i 

Sawdust 1 


140  STEEL  AND  ITS  HEAT  TREATMENT 

Parts 

6.  Coal  with  30  per  cent,  volatile  matter 5 

Charred  leather 5 

Salt 1 

Sawdust 15 

c.  Charred  leather 10 

Yellow  prussiate 2 

Sawdust : 10 

The  velocity  of  carburization  increases  gradually  from  the  first  to 
the  third  of  these  cements.  The  sawdust,  by  making  the  mass 
more  porous,  increases  the  activity  of  the  gases. 

Size  of  the  Carburizing  Box. — The  selection  or  general  design  of 
the  box  or  container  for  carburizing  is  worthy  of  more  attention  than 
is  frequently  given  to  it.  In  the  attempt  to  get  a  uniform  case,  much 
thought  and  research  has  been  given  to  the  selection  of  the  steel, 
the  carburizing  mixture  and  the  degree  and  duration  of  heating;  and 
yet  in  many  instances  it  has  all  proven  unavailing.  It  must  be 
remembered  that  the  inside  of  a  small  box  takes  quite  a  while  to 
come  up  to  the  temperature  of  the  furnace;  and  that  if  a  large  box 
is  used,  the  material  in  the  center  may,  and  does,  lag  behind  the 
indicated  furnace  temperature  several  hours  or  its  time  equivalent — 
several  hundred  degrees.  The  greater  the  size  of  the  box,  the  larger 
will  be  this  error,  and  the  greater  the  actual  difference  in  the  thick- 
ness of  case  taken  on  by  steel  near  the  sides  of  the  box  as  compared 
with  that  near  the  center  of  the  box.  No  manipulation  of  the  furnace 
can  change  this  effect;  it  can  only  be  remedied  by  altering  the 
dimensions  of  the  box  itself.  Here,  then,  lies  one  explanation  of 
many  unexplained  failures. 

The  box  should  not  be  larger  than  is  absolutely  necessary,  even 
where  large  quantities  are  to  be  carburized  in  it.  It  should  be  narrow 
in  at  least  one  dimension  so  that  the  heat  has  a  chance  to  penetrate 
quickly  at  least  from  two  sides  and  reach  all  the  contents  at  about 
the  same  time.  Further,  the  boxes  should  not  be  made  too  deep  in 
proportion  to  their  other  dimensions,  as  it  makes  it  more  difficult 
to  pack  the  parts  into  them  if  so  made.  Whenever  possible,  the 
design  of  the  box  should  follow  the  outline  of  the  piece  to  be  carbur- 
ized, allowing  about  1  to  2  ins.  all  around  for  clearance  and  packing, 
so  that  the  surfaces  may  be  uniformly  heated  and  carburized  alike. 

Material  for  Boxes. — Malleable  iron  probably  gives  as  good 
satisfaction  as  any  of  the  materials  used  in  making  the  boxes.  Cast- 


CASE  CARBURIZING  141 

iron  boxes,  although  of  comparatively  small  initial  cost,  will  not  stand 
reheating  very  many  times,  and  have  the  further  objectionable 
feature  of  being  somewhat  porous.  Soft-steel  plates  and  wrought 
iron  may  also  be  made  up  into  good  boxes. 

The  thickness  of  the  wall  forms  an  important  feature  of  the  box, 
for  if  it  is  too  thin  it  easily  burns  through,  and  if  too  thick  it  offers 
too  much  resistance  to  the  penetration  of  the  heat  to  the  interior. 
For  the  ordinary  size  boxes,  wall  thicknesses  of  J  to  J  in.  are  common 
practice.  The  boxes  should  be  provided  with  feet  so  that  the  heat 
may  circulate  all  around  them.  The  cover  should  be  as  close  fitting 
as  is  practicable,  and  should  also  be  provided  with  ribs  along  the  top 
to  prevent  excessive  warping.  Ribs  along  the  side  also  add  to  the 
service  of  the  box,  besides  making  handling  with  grappling  irons 
more  easy.  The  sides  of  the  boxes  should  taper  slightly  towards 
the  bottom  so  that  the  contents  can  be  the  more  quickly  dumped  out. 

Packing. — Carefulness  in  packing  is  fundamental  to  good  practice 
and  uniformity  of  results,  just  as  much  as  carefulness  in  heating  or 
treatment.  The  method  of  packing  should  be  such  as  will  insure 
as  nearly  as  possible  the  even  heating  and  uniform  carburizing  of  all 
pieces  in  the  same  box. 

The  method  of  packing  must  necessarily  vaiy  with  each  type 
of  article  to  be  handled.  Heavy  pieces,  or  pieces  of  regular  shape, 
do  not  require  the  care  and  patience  which  should  be  used  with  pieces 
of  intricate  design  or  with  those  which  on  account  of  their  size  and 
shape  may  be  readily  influenced  by  high  temperatures.  The  packing 
of  such  pieces  must  be  individualized.  For  example,  long,  slender 
pieces  should  always  be  packed  vertically,  so  that  the  pieces  will 
be  held  in  position  by  the  carburizing  material  and  cannot  sag  under 
the  influence  of  the  high  temperatures.  Again,  gears  and  similar 
pieces  may  be  most  suitably  packed  in  tubes,  so  that  the  same 
amount  of  carburizing  material  and  the  same  degree  and  length  of 
heating  may  influence  all  parts  of  the  periphery  in  equal  proportion. 
In  carburizing  screws  and  bolts  it  is  well  to  distribute  them  in 
the  box  in  two  opposite  rows,  each  row  having  the  head  of  the  screw 
towards  the  side  of  the  box  and  the  stem  towards  the  center.  New 
compound  may  be  used  at  the  sides  and  old  compound  in  the  center. 
Owing  to  the  difference  in  heat  and  the  difference  in  the  carburizing 
power  of  the  compounds,  this  will  cause  a  much  deeper  carburization 
of  the  heads  than  of  the  stems — which  is  exactly  what  is  desired. 
Again,  should  a  narrow  or  low  box  not  be  available  in  connection 
with  small  work,  carburizing  compound  which  has  been  used  once 


142  STEEL  AND   ITS  HEAT  TREATMENT 

before  may  be  put  next  to  the  sides  of  the  box  while  the  new  com- 
pound is  placed  in  the  center.  In  this  way  the  difference  in  car- 
burizing  which  might  result  from  the  different  temperatures  in 
various  parts  of  the  box  may  be  offset. 

The  first  step  in  the  general  operation  of  packing  is  to  cover  the 
bottom  of  the  box  with  the  compound  to  a  depth  of  1J  to  2  ins., 
tamping  it  solidly  into  place.  The  parts  to  be  carburized  are 
then  placed  firmly  upon  this  bed  so  that  the  compound  and  work 
are  in  close  contact  with  each  other.  The  pieces  should  in  no  case 
touch  the  sides  of  the  box,  but  should  be  placed  about  1  to  1J  ins. 
away  from  it.  Further,  the  articles  should  be  separated  from  each 
other  by  at  least  \  to  1  in.,  dependent  upon  their  size  and  the  depth 
of  case  desired.  If  the  articles  should  touch  one  another,  it  is  evident 
that  the  carburizing  action  will  have  less  influence  at  that  particular 
point — with  resulting  soft  spots.  Non -uniformity  of  case  may  also 
result  if  there  is  not  sufficient  carburizing  material  in  the  box;  it  is 
better  to  err  by  using  too  much  than  too  little. 

After  the  first  layer  of  work  has  been  placed  in  the  box,  it  is 
entirely  covered  with  the  carburizing  compound.  This  should 
be  packed  and  tamped  down  around  and  over  the  pieces  so  as  to 
have  the  particles  of  cement  in  close  contact  with  the  steel,  but  yet 
not  so  tightly  as  to  prevent  the  free  circulation  of  the  carburizing 
gases  which  are  generated  during  the  heating  process.  When  the 
first  layer  has  thus  been  suitably  packed  and  covered,  the  same 
procedure  is  repeated  until  the  box  is  nearly  filled.  The  point  to  be 
kept  in  mind  is  that  each  and  every  piece  should  be  surrounded  on 
all  sides  by  a  suitable  amount  of  the  carburizing  compound. 

At  least  2  ins.  of  the  compound  should  form  the  top  blanket  over 
the  last  layer  of  work.  Some  shops  adopt  the  following,  with  the 
aim  of  further  preventing  the  escape  of  the  gases :  about  2  ins.  from 
the  top  of  the  box  sheet-steel  strips  about  -^  in.  thick  are  laid  over 
the  last  layer  of  the  carburizing  material  and  these,  in  turn,  are 
covered  with  about  1  in.  of  powdered  charcoal.  When  the  box  is 
finally  packed,  the  cover  is  placed  on  the  box  and  the  edges  are  care- 
fully sealed  with  fire-clay  or  asbestos  cement.  The  box  is  now 
ready  for  the  heating  operation. 

Type  of  Furnace. — It  is  not  our  intention  to  recommend  any 
particular  type  of  furnace  for  carburizing  work,  but  rather  to 
emphasize  the  necessity  of  designing  the  furnace  to  suit  the  work. 
And,  as  is  evident,  the  conditions  will  vary  greatly  from  one  plant 
to  another. 


CASE   CARBURIZING  143 

There  are  three  main  points  which  should  be  taken  into  account 
for  case-hardening  furnaces.  (1)  The  furnace  shall  be  capable  of 
attaining  easily  the  maximum  temperature  which  shall  be  necessary 
for  the  carburizing  work,  and  which  temperature  may  be  as  high  as 
2000°  F.  (2)  It  must  be  possible  to  obtain  a  thoroughly  uniform 
heat  application  at  any  of  the  intervening  temperatures,  and  of 
maintaining  that  uniform  heating  with  little  or  no  variation  hour 
after  hour.  (The  effect  of  oscillating  temperatures  has  been  de- 
scribed.) (3)  The  atmosphere  in  the  furnace  shall  be  non-oxidizing, 
in  order  to  protect  the  carburizing  boxes  from  the  intense 
oxidation  which  would  otherwise  occur  at  the  high  temperatures 
necessary. 

The  Heating. — The  two  principal  points  to  be  mentioned  under 
this  heading  are:  the  heating,  at  least  up  to  1300°  F.,  should  be 
gradual;  (2)  the  heating  beyond  this  temperature  should  be  uniform 
over  all  parts  of  the  carburizing  box.  It  has  been  shown  by  several 
experimenters  that  the  energetic  liberation  of  gases  commences 
very  strongly  at  temperatures  somewhat  under  1300°  F.  for  the 
majority  of  solid  cements,  and  it  is  advisable  to  diminish  this  factor 
as  much  as  possible  in  order  to  obtain  a  more  gradual  cementation. 
Furthermore,  it  gives  more  opportunity  for  the  steel  to  adjust  itself 
to  the  effect  of  heating.  The  second  point  made  is  self-evident: 
non-uniformity  of  heating  must  necessarily  result  in  non-uniformity 
of  product. 

Sulphur  Diffusion. — The  influence  of  sulphur  contained  in  the 
cements  is  an  extremely  important  factor  in  carburization  carried 
on  with  solid  cements.  Grayson  l  has  produced  uncontrovertible 
evidence  that  sulphur  will  diffuse  into  iron  at  the  temperatures 
ordinarily  used  for  carburization  with  such  substances  as  charred 
leather  (which,  under  the  conditions  of  his  case-hardening  experi- 
ments, contained  0.55  per  cent,  total  sulphur),  and  that  this  sulphur 
combines  with  the  manganese  and  iron  to  form  manganese  and  iron 
sulphides. 

Thus  in  Fig.  77,  which  is  a  photomicrograph  of  a  piece  of  0.17 
per  cent,  carbon  steel  carburized  for  six  hours  at  1650°  to  1750°  F. 
with  charred  leather,  it  will  be  noticed  that  on  the  edge  are  present, 
in  large  quantities,  sulphide  of  manganese,  also  sulphide  of  iron  with 
ferrite  crystals  intermingled.  That  this  is  sulphide  was  later  proven 
by  means  of  silver  prints  and  by  analysis — which  showed  2.10  per 
cent,  of  sulphur  increase  in  the  first  0.0025  inch. 

!S.  A.  Grayson,  Inst.  Journ.,  No.  1,  1910. 


144 


STEEL  AND  ITS  HEAT  TREATMENT 


This  sulphur  diffusion  is  a  very  serious  matter,  because  when  the 
surface  is  saturated,  as  in  this  figure,  it  tends  to  produce  a  soft  skin, 


FIG.  77.— Soft  Case  Due  to  Sulphur 
Diffusion.     (Grayson.) 


• 


FIG.  78.  —  Sulphides  Diffusing 
Further  into  Case  with  Higher 
Temperatures  of  Carburization. 
(Grayson.) 


FIG.  79. — Sulphide  Globules  in  Carburized  Steel  after  Hardening.     (Grayson.) 

and  even  if  present  in  smaller  proportions  it  will  weaken  the  structure 
considerably,  thus  making  it  very  "  chippy,"  consequently  causing 


CASE  CARBURIZING 


145 


two  effects  which  must  essentially  be  avoided  in  any  case-hardened 
work. 

In  Fig.  78,  being  a  similar  steel  carburized  at  1750°  to  1830°  F., 
the  sulphide  is  again  present,  but  not  in  such  a  large  proportion; 
thus  the  higher  temperature  has  volatilized  still  more  of  the  sul- 


FIG.  80. — American  Gas  Furnace  Co.'s  Carburizing  Machine. 

phur  from  the  carburizing  material.  Fig.  79  shows  the  same  car- 
burized piece  as  in  Fig.  77,  but  afterwards  reheated  and  quenched 
in  water  from  1380°  F.  In  this  reheating  the  sulphide  tends  to 
"  ball  "  itself  up,  and,  if  anything,  diffuse  further  in. 

Thus  it  may  be  seen  that,  for  proper  carburizing,  the  solid 
cements  should  be  as  free  from  sulphur  as  is  possible.     On  the  other 


146 


STEEL  AND   ITS  HEAT  TREATMENT 


*b   f 


CASE   CARBURIZING 


147 


hand,  the  barium  carbonate  mixtures  generally  used  do  not  contain 
sulphur,  and  this  sulphur  diffusion  cannot  take  place. 

American  Gas  Furnace  Process. — The  apparatus  for  carburizing 


with  gas,  as  devised  by  the  American  Gas  Furnace  Company,  is 
shown  in  Fig.  80.  The  carburizing  machine  consists  of  a  carburizing 
retort  enclosed  by  a  cylindrical  furnace  body  in  which  it  rotates, 


148  STEEL  AND   ITS  HEAT   TREATMENT 

together  with  suitable  arrangements  for  charging  and  discharging 
the  work,  burners  for  securing  a  proper  distribution  of  the  fuel,  and 
supply  pipes  for  gas  and  air.  The  machine  shown  has  a  space 
available  for  work  of  30  ins.  in  length  by  7  ins.  in  diameter.  It  is 
suitable  for  work  not  over  6  ins.  in  diameter  or  20  ins.  in  length;  for 
shafts,  tubes,  mandrels  and  bars  of  nearly  equal  thickness  through- 
out of  not  over  24  ins.  in  length  or  5  ins.  in  diameter;  or  for  small 
pieces  such  as  screws,  washers,  discs,  etc.,  of  a  charge  of  about  100 
pounds.  The  machine  uses  ordinary  illuminating  gas  for  both  heat- 
and  carburizing. 

The  vertical  section,  Fig.  81,  through  the  center  lengthwise, 
shows  the  heavy  wrought-iron  retort  A,  which  is  slowly  rotated  on 
the  rollers  BB  by  the  gear  C,  in  contact  with  worm  D,  propelled  by 
a  sprocket  and  chain  belt.  The  reference  letters  EE  show  air  spaces 
in  the  retort  formed  by  the  two  pistons  /,  between  which  the  work 
is  confined,  to  the  properly  heated  central  section  of  the  retort. 
Letters  FF  indicate  the  heating  space  surrounding  the  retort,  into 
which  the  fuel  gas  and  air  are  injected  under  pressure,  from  two  rows 
of  burners  indicated  in  the  upper  half  of  the  casing  by  the  letter  G. 
The  cover  H,  closing  the  retort,  is  connected  with  the  piston-like 
disc  marked  /,  by  the  pipe  J,  which  is  the  vent  of  the  retort.  The 
cover  H  and  disc  I  are  withdrawn  to  charge  the  retort  and  replaced 
after  the  work  is  inserted. 

Carburizing  machines  connected  with  an  automatic  quenching 
bath  are  shown  in  Fig.  82. 

SUPERFICIAL   HARDENING 

Superficial  hardening  differs  from  case  carburizing  in  that  in 
the  former  method  the  outer  and  higher  carbon  section  constitutes 
a  "  skin  "  of  only  a  few  thousandths  of  an  inch  in  thickness,  while 
in  the  case-carburizing  process  the  carburized  zone  forms  a  case 
of  noticeable  thickness.  Exactly  the  same  principles  apply,  however, 
in  both  instances,  and  which  have  been  previously  explained. 

Processes. — The  superficial  hardening  processes  may  be  grouped 
under  the  headings  of  "  cyanide  hardening  "  and  "  pack  hardening.  " 

The  cyanide  hardening  processes  are  essentially  used  for  the  pur- 
pose of  obtaining  an  extreme  degree  of  surface  hardness  (wear)  on 
low-carbon  or  machinery  steel,  and  in  which  it  is  not  necessary  to 
obtain  high  resistance  to  shock,  etc. 

On  the  other  hand,  pack  hardening  is  essentially  a  method  of 
heating  used  particularly  for  fine  threaded  tools  and  other  tool- 


CASE  CARBURIZING  149 

steel  work.  The  process,  when  correctly  carried  out,  permits  of 
uniform  heating  with  the  entire  elimination  of  oxidation  by  surround- 
ing the  steel  with  a  carbonaceous  packing.  But  further,  by  prolong- 
ing the  duration  of  heating  at  the  hardening  temperature,  a  very 
thin  skin  of  higher  carbon  content  may  be  formed,  so  that  pack 
hardening  may  develop,  either  intentionally  or  otherwise,  into  a 
superficial  hardening  process. 

Cyanide  Hardening. — In  cyanide  hardening  the  superficial  car- 
burizing  and  hardening  may  be  effected  by  one  of  two  general 
methods:  (1)  immersion  of  the  object  in  a  bath  of  liquid  potassium 
cyanide  or  other  mixture  with  cyanogen  as  the  base,  followed  by 
quenching;  (2)  coating  or  sprinkling  the  surface  of  the  object  with  an 
adhesive  mixture  of  finely  pulverized  carburizing  cyanogenous  salt 
or  "  varnish,"  heating  the  steel  to  the  proper  hardening  tempera- 
ture— and  thus  melting  the  cyanide — and  hardening  as  usual. 
The  first  or  "  immersion  "  process  is  by  far  the  most  efficient,  both 
as  to  unifcrmity  of  the  carburized  zone  and  simplicity  and  uniformity 
of  operation.  Further,  this  first  method  has  the  tendency  to  reduce 
deformation  and  oxidation  during  heating  and  quenching,  since, 
as  previously  explained,  heating  in  any  molten  bath  has  this  effect. 

The  Immersion  Method. — The  method  of  cyanide  hardening  by 
immersion  is  quite  simple.  The  salt,  usually  potassium  cyanide 
(KCN),  is  melted  in  a  suitable  pot-furnace,  and  is  maintained  at  a 
temperature  a  little  over  the  upper  critical  range  of  the  steel  to  be 
carburized  and  hardened.  This  temperature,  for  ordinary  machin- 
ery steel,  is  about  1550°  to  1600°  F.  The  steel  is  then  immersed 
in  the  molten  cyanide  and  kept  there  until  it  has  been  uniformly 
heated;  or  this  heating  may  be  somewhat  prolonged  in  order  to 
obtain  a  greater  depth  of  skin.  In  general,  however,  it  is  not  advis- 
able to  heat  for  a  length  of  time  much  greater  than  ten  or  fifteen 
minutes,  or  at  temperatures  much  over  the  critical  range,  since  such 
heating  will  tend  to  give  non-uniform  and  high-carbon  zones  which, 
after  quenching,  are  intensely  brittle  and  may  chip  off  in  service. 
Quenching  is  usually  done  in  lime  water  in  order  to  neutralize  the 
cyanide  remaining  on  the  steel.  Some  concerns  adopt  the  method 
of  immersing  the  steel  in  the  cyanide  as  soon  as  it  has  become  molten, 
permitting  the  steel  to  heat  up  with  the  bath,  and  then  quenching 
as  soon  as  the  desired  temperature  of  say  1550°  to  1575°  F.  has  been 
attained. 

It  is  absolutely  necessary  to  remember  that  cyanogen  compounds 
are  deadly  poisonous,  and  every  precaution  should  be  adopted  when 


150 


STEEL  AND   ITS  HEAT  TREATMENT 


using  them.  Furnaces  should  be  supplied  with  hoods  which  have 
strong  draft.  Gloves  should  be  used  in  handling  all  work,  for  if 
cyanide  gets  into  a  fresh  cut  or  scratch  it  will  prove  deadly.  In 
some  cases,  when  working  at  the  furnaces,  it  is  even  advisable  to 
use  face  masks  and  to  cover  up  any  exposed  parts  of  the  body. 

Cyanide  Hardening    Plant. — A  battery  of  twenty  cyanide  fur- 
naces is  shown  in  Figs.  83  and  84. l     In  front  of  the  first  pair  of 


FIG.  83. — Battery  of  Cyanide  Furnaces' — Special  Quenching    Machines  for 
Clutch  Rings  in  Foreground.     ("Machinery.") 


furnaces  in  Fig.  83  two  special  machines  are  shown  which  suddenly 
cool  or  quench  the  work  as  fast  as  it  can  be  heated  and  removed 
from  the  furnace.  They  are  used  for  hardening  the  steel  ring  discs 
shown  at  U.  These  alternate  with  brass  discs  in  a  multiple-disc 
clutch  on  the  engine  of  an  automobile.  Each  pair  of  furnaces  shown 
in  these  two  figures  is  covered  with  a  hood  to  convey  the  poisonous 
fumes  to  the  outer  atmosphere  through  pipes  extending  through  the 
roof.  In  addition  to  this,  sheet-metal  shields  are  located  in  front  of 

1  E.  F.  Lake,  in  "  Machinery,"  Sept.,  1914. 


CASE  CARBURIZING 


151 


the  furnace  openings  shown  at  V  to  carry  away  from  the  workmen 
any  fumes  that  might  come  through  these  openings.  (These  shields 
were  removed  for  photographing.)  At  the  end  of  the  cyanide  fur- 


o 

b£ 

I 


I 
I 
1 


<y 


naces  shown  in  Fig.  84  is  a  stationary  tank  of  lime  water  in  which  some 
of  the  work  is  quenched.  On  the  floor  is  shown  a  tray  loaded  with 
bevel  differential  gears  and  having  a  long  rod  for  a  handle.  This  is 
lowered  into  the  cyanide  bath  to  heat  the  gears,  then  lifted  out  and 


152  STEEL  AND  ITS  HEAT  TREATMENT 

lowered  into  the  quenching  tank,  and  when  cool  the  gears  are  dumped 
into  boxes  to  take  to  the  tempering  furnaces.  Other  parts  that  are 
being  hardened  are  shown  in  the  metal  boxes  beside  the  tray  of  gears. 

Other  Cyanide  Methods. — The  second  general  process  of  cyanide 
hardening,  in  its  simplest  form,  is  to  heat  the  steel  to  about  1550°  F. 
or  so;  sprinkle  upon  it,  or  plunge  it  into,  potassium  cyanide  or 
potassium  ferrocyanide;  again  heat  to  1550°  to  1600°  F.  until  the 
cyanide  is  melted;  and  then  quench  in  water.  In  case  the  amount 
of  cyanide  obtained  the  first  time  is  insufficient,  the  operation  previ- 
ous to  quenching  may  be  repeated  until  a  layer  of  the  required  thick- 
ness is  obtained.  It  is,  of  course,  necessary  to  have  a  clean  surface, 
free  from  scale  and  oxidation,  so  that  the  carburizing  reactions  may 
take  place  readily. 

Other  more  elaborate  processes  based  upon  the  above  are  in  use, 
and  involve  the  application  of  special  carburizing  varnishes.  On  the 
whole,  however,  the  simpler  the  process  or  carburizing  compound, 
the  more  efficacious  will  it  be  in  actual  and  everyday  practice. 

Pack  Hardening. — Pack  hardening,  as  a  superficial  carburizing 
process,  so  raises  the  carbon  content  in  the  surface  of  the  steel  that 
the  tools  may  be  hardened  in  oil — instead  of  in  water — and  still 
obtain  the  requisite  degree  of  either  cutting  or  wearing  hardness. 
For  certain  work  requiring  almost  perfect  hardening  results  this 
method  cannot  be  overestimated.  In  cases  in  which  the  required 
degree  of  hardness  may  usually  be  obtained  only  by  the  use  of  water 
quenching,  oil  quenching  may  now  be  used;  and  with  it  will  be 
associated  the  toughness  of  core  inherent  with  the  use  of  oil  as  a 
quenching  medium.  Further,  on  account  of  the  uniformity  in  heat- 
ing and  the  use  of  oil  quenching,  the  tendency  to  crack  or  warp  is 
largely  eliminated. 

The  method  is,  in  fact,  a  case-carburizing  process.  The  packing 
in  boxes  is  carried  out  in  exactly  the  same  manner  as  is  carburization 
with  solid  cements,  and  similar  precautions  should  be  used  to  prevent 
the  tools  being  jarred  out  of  position  or  touching  each  other.  In 
pack  hardening,  however,  to  each  tool  or  piece  of  steel  should  be 
attached  a  wire,  so  that  the  tool  may  be  removed  promptly  to  the 
quenching  bath  when  the  requisite  degree  and  duration  of  heat  has 
been  attained. 

The  temperature  to  be  used  should  be  but  slightly  over  the 
critical  range  of  the  steel,  thus  differing  from  the  higher  temper- 
atures which  are  customary  in  case-carburizing  processes.  As  the 
pack-hardening  process  is  usually  used  for  steels  of  tool-steel  analysis, 


CASE  CARBURIZING  153 

this  temperature  will  be  about  1375°  to  1400°  F.  The  length  of 
time  required  for  the  heating  will,  of  course,  depend  upon  the  size 
and  number  of  the  pieces  to  be  treated  in  one  box,  and  the  depth  of 
skin  desired;  for  ordinary  small  tools  this  will  generally  be  about 
two  hours  after  the  proper  temperature  has  been  attained  by  the 
steel  itself. 

Packing  material  which  would  be  harmful  to  the  steel  should  not 
be  used.  Bone,  for  example,  usually  contains  phosphorus,  which  is 
apt  to  make  the  steel  brittle — although  burnt  bone  is  not  as  high  in 
this  element  as  is  raw  bone.  Sulphur  must  also  be  guarded  against. 
If  the  initial  steel  does  not  contain  more  than  1.20  or  1.25  per  cent, 
carbon,  charred  leather  makes  a  very  good  packing  material.  If 
the  carbon  content  exceeds  these  values,  charred  hoofs,  or  a  mixture 
of  charred  hoofs  and  horns  is  better  than  charred  leather,  since  the 
latter  will  under  such  conditions  have  the  tendency  to  give  a  too 
highly  carburized  and  brittle  zone. 

The  temperature  of  the  steel  in  the  box  may  be  gauged  by  means 
of  test  rods  the  same  size  as  the  tools,  or  by  test  wires,  or  by  suitable 
pyrometer  equipment. 


CHAPTER  VII 
CASE   HARDENING:   THERMAL   TREATMENT 

Heat-treatment  Requirements.— It  may  be  said  that  practically 
all  objects  which  have  undergone  the  case-car burizing  processes 
previously  described  require  a  subsequent  heat  treatment  of  some 
nature.  As  one  of  the  essential  aims  of  the  case-hardening  process 
is  to  produce  a  hard-wearing  surface,  and  as  carburized  steels  through 
their  slow  cooling  from  high  temperatures  will  be  more  or  less  lack- 
ing in  this  necessary  hardness,  it  is  evident  that  a  hardening  process 
is  necessary.  In  dealing  with  the  subject  of  case  hardening  we  will 
therefore  assume  that  the  carburized  steel  must  undergo  some 
hardening  process  or  processes  which  will  bring  about  this  desired 
condition  of  affairs. 

Secondly,  in  order  that  we  may  at  once  differentiate  the  ultimate 
aims  of  such  hardening,  and  simplify  our  discussion,  we  will  assume 
that  we  also  desire  to  obtain  a  minimum  brittleness  in  both  case  and 
core.  Previous  explanations  prove  that  this  condition  requires  that 
the  "  grain  size  "  be  reduced  to  a  minimum,  that  is,  that  the  steel 
as  a  whole  must  be  refined. 

To  sum  up,  the  specific  aims  which  we  have  in  view  require  that 
the  heat  treatment  shall  combine  hardening  and  grain  refinement. 

Comparison  with  Homogeneous  Steels. — The  heat  treatment  of  a 
carburized  steel  differs  from  that  of  a  homogeneous  steel  only  in  the 
fact  that,  instead  of  considering  the  influence  of  such  treatment 
upon  one  steel,  we  really  have  to  do  with  two,  or  even  three  main 
classes  of  steels  at  once.  That  is,  the  carburized  steel  consists  of 
(1)  the  core,  or  low-carbon  steel;  (2)  the  carburized  zones  of  the  case 
with  about  0.9  per  cent,  carbon  as  the  maximum;  and,  in  many 
instances,  (3)  the  carburized  zones  of  the  case  which,  under  conditions 
of  slow  cooling  from  the  temperature  of  carburization,  contain  an 
excess  of  free  cementite,  i.e.,  greater  than  0.9  per  cent,  carbon.  Our 
heat  treatment  must  therefore  be  adjusted  so  as  to  superimpose 
the  effect  of  one  class  upon  the  other. 

Now  as  the  present  tendency  of  case  carburizing  in  industrial 
practice  is  to  preclude  the  formation  of  zones  containing  free  cement- 

154 


CASE  HARDENING:    THERMAL  TREATMENT  155 

ite  (class  3),  we  will  postpone  the  discussion  of  the  heat  treatment 
which  involves  that  class,  and  thus  further  simplify  matters.  We 
now,  therefore,  have  but  to  consider  the  related  heat  treatment  of 
steels  of  very  low-carbon  content  and  those  containing  the  eutectoid 
ratio  of  carbon  as  a  maximum. 

Effect  of  the  Temperature  of  Carburization. — Into  this  heat 
treatment  there  now  enters  the  factor  of  the  temperature  of  car- 
burization,  and  its  specific  influence  upon  the  size  of  grain  in  the 
two  classes  of  steels.  In  the  first  place,  it  is  axiomatic  that  the 
effect  of  any  heating  at,  or  slightly  above,  the  Ac3  range  for  the  soft 
steels,  and  the  Acl.2.3  range  for  the  hard  steels,  is  to  produce  the 
maximum  grain  refinement  (unless  such  heating  is  extremely  pro- 
longed) .  And  further,  that  the  effect  of  any  heating  at  temperatures 
considerably  above  these  temperatures  is  to  produce  a  size  of  grain  of 
proportionally  greater  size  for  the  respective  steels.  (In  this  chapter 
we  are  using  the  phrase  "  grain  size  "  in  its  general  colloquial  mean- 
ing.) Thus,  if  carburization  were  carried  out  at  1600°  F.  for  a  case 
carburizing  steel  of  0.15  per  cent,  carbon — that  is,  at  a  temperature 
but  slightly  over  that  of  the  upper  critical  range  of  the  initial  steel — 
it  would  produce  the  minimum  grain  size  in  the  steel  of  the  core, 
and  a  certain  and  proportionally  greater  grain  size  in  the  steel  of  the 
case.  Steels  carburized  at  the  lower  temperatures  we  will  call 
Group  A.  Steels  carburized  at  the  higher  temperatures  or  about 
1800°  F. — that  is,  considerably  over  that  of  the  upper  critical  range — 
we  will  call  Group  B,  the  grain  size  of  both  core  and  case  being 
proportionally  greater  than  the  minimum. 

Classification. — We  now  have  a  further  means  of  classifying  our 
heat-treatment  processes  according  to  the  temperature  which  was 
used  in  carburization  because  of  the  effect  of  such  temperatures  on 
the  refinement  of  the  steel.  That  is,  with  carburizations  of  Group 
A,  the  steel  of  the  core  will  already  be  refined,  and  we  need  only 
consider  the  refining  of  the  case;  while  in  Group  B  the  steel  of  both 
core  and  case  will  require  refinement.  Both  groups  will,  of  course, 
require  the  hardening  of  the  case. 

Treatment  of  Group  A. — Assuming  the  conditions  as  in  Group  A 
(carburization  at  a  temperature  slightly  over  the  upper  critical  range 
of  the  initial  steel),  it  is  evident  that  the  complete  heat  treatment 
following  carburizing  will  only  require  the  hardening  and  refining 
of  the  case,  in  which,  by  previous  assumption,  the  maximum  car- 
bon content  is  about  0.9  per  cent.  A  consideration  of  the  principles 
of  heat  treatment  at  once  shows  us  that  a  single  quenching  at  about 


156  STEEL  AND  ITS  HEAT  TREATMENT 

1375°  F.,  that  is,  slightly  over  the  Al.2.3  range,  will  bring  about  the 
fulfillment  of  both  of  these  conditions.  Further,  the  quenching  at 
this  temperature,  and  under  existing  conditions,  will  not  affect  the 
present  refinement  of  the  core,  nor — if  the  carbon  content  is  low — 
will  it  in  crease  the  brittleness  due  to  the  changing  of  the  pearlite 
of  the  core  into  martensite  (in  fact,  it  has  the  opposite  effect  of  in- 
creasing the  toughness  in  the  very  low  carbon  steels) .  Thus,  by  this 
single  quenching,  we  have  completed  the  requirements  originally 
demanded. 

Treatment  of  Group  B. — Turning  now  to  case  carburizations  at 
the  higher  temperatures,  Group  B,  it  is  evident  that  in  addition  to 
the  refining  and  hardening  of  the  case  we  must  also  refine  or  regener- 
ate the  core.  This,  we  know,  may  be  best  accomplished  by  quench- 
ing the  previously  cooled  steel  at  a  temperature  slightly  above  the 
Ac3  range  of  the  steel  of  the  core.  This  quenching  will  put  the  entire 
steel,  both  case  and  core,  in  the  martensitic  condition,  refine  the 
core,  but  not  refine  the  case.  By  following  this  first  quenching  by 
the  quenching  at  the  lower  temperature  described  in  the  previous 
paragraph  we  accomplish  the  following :  the  hardness  and  refinement 
of  the  case  reaches  a  maximum;  the  refinement  of  the  core  produced 
by  the  first  or  regenerative  quenching  is  not  changed;  the  strains 
or  brittleness  which  may  have  been  produced  in  the  core  through 
the  first  quenching  are  relieved.  In  other  words,  by  superimposing 
one  quenching  upon  the  other  we  have  attained  the  desired  prop- 
erties. 

Effect  of  Hyper-Eutectoid  Zone. — The  next  variable  is  that  due 
to  a  hyper-eutectoid  zone  in  the  case,  or  carburized  steels  which  con- 
tain free  cementite  upon  slow  cooling  from  the  temperature  of  car- 
burization.  If  we  apply  the  treatment  previously  described  under 
Group  A  the  condition  of  this  free  cementite  will  not  be  affected. 
This  is  due  to  the  fact  that,  upon  heating,  this  free  cementite  is  not 
dissolved  by  the  solid  solution  austenite  until  a  temperature  corre- 
sponding to  the  Acm  range  (see  Fig.  13)  of  the  maximum  carbon 
content  of  the  case  is  attained,  and  which  is  obviously  higher  than 
1375°  F. 

Influence  of  Free  Cementite. — Now  it  has  been  repeatedly 
demonstrated  in  practice  that  the  presence  of  free  cementite  existing, 
as  it  usually  does,  in  the  form  of  films  between  the  grains  (i.e.,  as  a 
network),  or  even  as  spines,  increases  the  brittleness  of  the  case, 
interposes  lines  of  weakness,  and  often  results  in  the  chipping  off 
of  parts  of  the  case.  (Incidentally  it  might  be  mentioned  that  this 


CASE  HARDENING:    THERMAL  TREATMENT  157 

is  another  reason  for  desiring  a  maximum  carbon  content  in  the  case 
of  about  0.9  per  cent,  when  suitably  adjusted  and  controlled  methods 
of  heat  treatment  are  not  used.)  To  eliminate  this  source  of  dan- 
ger — the  free  cementite — it  will  be  necessary  to  heat  the  steel  above 
the  Acm  range  of  the  steel  of  the  case  in  order  to  get  this  cementite 
"  into  solution,"  and  to  then  "  fix  "  it  in  that  condition  by  quench- 
ing from  that  temperature.  Now,  provided  that  the  maximum  carbon 
content  is  not  sufficiently  high  so  as  to  raise  this  Acm  range  above  the 
Ac3  range  of  the  steel  of  the  core,  it  is  apparent  that  the  treatment 
of  Group  B  previously  noted  (the  double  quenching)  will  also  serve 
in  this  instance.  This  treatment  will  likewise  be  applicable,  with  the 
above  proviso,  regardless  of  the  temperature  of  carburization. 


•'  * 


& 


V  • 

*  " 


7^-'  *  (  ,«t 


FIG.  85.— Core  of  Steel  Carburized  at  1830°  F.  and  Slow  Cooled.     (Bullens.) 

Photomicrographic  Study. — The  principles  brought  out  by  this 
series  of  treatments  and  their  individual  effect  on  the  case  and  core 
may  be  more  graphically  illustrated  by  means  of  the  series  of  photo- 
micrographs shown  in  Figs.  85  et  seq.  The  steel  in  these  photo- 
micrographs represents  an  ordinary  low-carbon  steel  which  has  been 
cemented  at  1830°  F.  in  such  a  manner  as  to  produce  a  carburized 
zone  containing  greater  than  0.9  per  cent,  carbon.  In  all  cases  the 
steel  has  been  allowed  to  cool  slowly  from  the  temperature  of  cemen- 
tation. 

Structure  after  Slow  Cooling. — The  micro-structure  of  the  core 
upon  slow  cooling  is  shown  in  Fig.  85,  it  consisting  of  coarse  ferrite 


158 


STEEL  AND   ITS  HEAT  TREATMENT 


(light)  and  a  small  amount  of  coarse  pearlite  (dark).     Similarly,  the 
micro-structure  of  the  external  layers  of  the  case  is  illustrated  in 


FIG.  86.— Case  of  Steel  Carburized  at  1830°  F.,  and  Slow  Cooled.     (Bullens.) 


FIG.  87. — Core  of  Steel  Carburized  at  1830°  F.,  Slow  Cooled,  and  Quenched  from 

1375°  F.     (Bullens.) 

Fig.  86,  in  which  it  is  seen  that  the  laige  grains  of  sorbitic  pearlite 
are  surrounded  by  the  characteristic  network  structure  of  free 
cementite.  In  other  words,  the  steel  as  a  whole  exhibits  the  non- 


CASE   HARDENING:    THERMAL  TREATMENT 


159 


refinement  characteristic  of  the  high  temperature  of  carburization, 
and  the  case  is  further  weakened  by  the  presence  of  free  cementite. 

Effect  of  Lower  Quenching  on  the  Core. — If  we  should  now 
quench  the  steel  from  about  1375°  F.,  we  see  from  Fig.  87  that  the 
effect  upon  the  core  is  to  change  the  pearlite  into  martensite  plus 
osmondite  (the  darker  areas),  to  slightly  increase  it  in  amount  (on 
account  of  the  fact  that  this  quenching  temperature  is  somewhat 
above  the  Al  range),  but  does  not  give  any  great  amount  of  grain 
refinement  to  the  core  as  a  whole. 


FIG.  88. — Case  of  Steel  Carburized  at  1830°  F.,  Slow  Cooled,  and  Quenched  from 

1375°  F.     (Bullens.) 

Effect  of  Lower  Quenching  on  the  Case.  Similarly  we  see  from 
Fig.  88,  representing  the  micro-structure  of  the  hardened  high-carbon 
case,  that,  although  the  initial  pearlite  itself  (compare  with  Fig.  86) 
has  been  refined,  as  well  as  changed  into  hard  martensite,  the  cement- 
ite network  has  remained  unaffected.  This  last  consequently 
causes  the  original  coarse  structure  of  the  case  as  a  whole  to  be 
retained  (that  is,  unrefined),  as  well  as  the  inherent  brittleness  due 
to  this  free  cementite. 

Regeneration  of  the  Core. — Now  by  quenching  the  steel  from  a 
temperature  just  above  the  upper  critical  range  of  the  steel  of  the 


160  STEEL  AND  ITS  HEAT  TREATMENT 

core,  or  at  about  1650°  F.,  we  see  from  Fig.  89  that  the  core  consists 
entirely  of  homogeneous  martensite,  and  further,  that  the  former 
coarse  grain  has  been  entirely  obliterated.  In  other  words,  we  have 
"  regenerated  "  the  core. 

Effect  of  Regenerative  Quenching  on  the  Case. — The  effect  of 
this  same  regenerative  quenching  upon  the  high-carbon  case  is 
shown  in  Fig.  90.  From  this  photomicrograph  it  is  evident 
that,  although  we  have  effected  a  rearrangement  and  entangling 
of  the  cementite  (white)  and  thus  largely  reduced  the  weaken- 
ing and  embrittling  effect  of  the  free  cementite  (as  in  Fig,  86),  the 


FIG.  89.— Core  of  Steel  Carburized  at  1830°  F.,  Slow  Cooled,  and  Quenched  from 

1650°  F.    (Bullens.) 


heating  and  quenching  temperature  of  1650°  F.  has  not  been  suffi- 
cient to  dissolve  entirely  and  "  fix  "  the  cementite  in  the  martensite. 
It  is  at  once  apparent  that  in  this  particular  steel  we  have  ex- 
ceeded the  proviso  regarding  the  maximum  carbon  content  which 
we  enunciated  in  a  previous  paragraph. 

Treatment  of  High-carbon  Case. — This  leads  to  a  consideration 
of  what  method  we  shall  apply  when  the  maximum  carbon  con- 
tent of  the  case  exceeds  that  percentage  which  will  cause  the  Acm 
range  to  be  above  the  Ac3  range  of  the  steel  of  the  core.  So  in  this 
particular  instance  we  have  two  procedures  open  to  us:  we  may  either 


CASE  HARDENING:    THERMAL  TREATMENT  161 

proceed  with  the  quenching  at  1650°  F.,  obtain  the  best  possible 
refinement  of  the  core,  and  accept  with  as  good  grace  as  we  can  the 
presence  of  free  cementite  in  the  final  case — granting  that  it  is  better 
distributed  by  this  quenching  than  by  the  treatment  as  in  Fig.  88; 
or,  we  may  raise  the  temperature  of  the  initial  quenching  to  such 
a  temperature  as  will  completely  dissolve  and  fix  the  excess  cementite, 
even  though  it  does  increase  the  grain  size  (and,  therefore,  the  brittle- 
ness)  of  the  core.  In  either  procedure  it  will  of  course  be  necessary 
to  follow  the  initial  quenching  by  the  hardening  quenching.  The 
proposition  then  comes  to  the  point  as  to  which  is  the  more  impor- 


FIG.  90.— Case  of  Steel  Carburized  at  1830°  F.,  Slow  Cooled,  and  Quenched  from 

1650°  F.     (Bullens.) 


tant,  (1)  the  greatest  refinement  of  the  core,  or  (2)  the  most  advan- 
tageous treatment  for  the  case,  such  as  will  give  minimum  brittleness, 
minimum  possibility  for  enfoliation  to  occur,  and  the  best  wearing 
surface. 

Effect  of  Very  High  Quenching  Temperature  on  the  Core. — 
Assuming  that  the  higher  temperatures,  which  will  be  necessary  if 
the  second  item  is  to  predominate,  can  be  used  without  the  oxidation 
of  the  steel  (such  as  by  the  use  of  salt  baths) ,  excessive  warping,  and 
so  forth,  a  study  of  the  photomicrograph  of  Fig.  91  will  aid  in  solving 
the  problem.  This  figure  represents  the  structure  of  the  core  after 
quenching  at  1830°  F.,  followed  by  a  second — the  "  hardening  " — 


162  STEEL  AND  ITS  HEAT  TREATMENT 

quenching  from  1375°  F.  It  is  at  once  manifest  that  the  high  quench- 
ing heat  has  not  greatly  increased  the  grain  size  of  the  core.  And 
further,  as  the  ferrite  and  sorbite  have  been  distributed  over  the  whole 
section  in  fine  particles,  the  core  should  prove  very  tough  on  this 
account.  That  is,  such  treatment  will  generally  prove  satisfactory 
for  the  core  so  long  as  the  initial  carbon  content  is  not  too  high, 
and  we  may  proceed  along  the  lines  which  shall  produce  the  best 
case. 

Treatment  for  "  Best  Case." — What,  now,  is  the  best  case — 
in  other  words,  the  best  wearing  surface;  and  how  may  it  be  obtained? 
From  previous  discussion,  and  from  a  study  of  the  photomicro- 


Fio.  91.— Core  of  Steel  Carburized  at  1830°  F.,  Slow  Cooled,  and  Double 
Quenched  from  1830°  and  1375°  F.     (Bullens.) 


graphs  of  this  and  the  preceding  chapter,  it  must  be  evident  that  the 
best  wearing  surface,  all  things  considered,  is  not  characterized  by 
the  presence  of  free  cementite  as  a  network  or  as  spines.  Granting 
this,  the  way  by  which  this  condition  may  be  avoided,  assuming  that 
the  carburized  steel  contains  a  hyper-eutectoid  zone,  is  first  to 
eliminate  the  free  cementite  by  quenching  above  the  Acm  range  of 
the  case,  and  to  be  -followed  by  a  treatment — for  purposes  of  grain 
refinement  and  hardening  of  the  case — such  as  will  not  reproduce 
the  original  network  condition  of  the  free  cementite.  Of  necessity, 
for  reasons  previously  given,  this  second  operation  must  consist  of 
a  quenching  at  about  1375°  F.  for  straight  carbon  steels. 


CASE  HARDENING:    THERMAL  TREATMENT  163 

Effect  of  Double  Quenching  on  the  High-carbon  Case. — The 

effect  of  these  two  quenching  operations  is  shown  in  the  photo- 
micrograph of  Fig.  92.  The  steel  contained  about  1.40  per  cent, 
carbon  and  was  quenched  from  about  1850°  F.,  followed  by  another 
quenching  at  1375°  F.  In  this  instance  it  will  be  noted  that  the 
free  cementite  appears  as  white  dots  or  "  spheroids  "  upon  the  darker 
martensitic  groundmass,  and  that  there  is  not  the  slightest  appear- 
ance of  the  originally  characteristic  network  structure  of  free 
cementite.  If  the  temperature  of  the  initial  quenching  has  not 
been  sufficiently  high  so  as  to  dissolve  all  the  original  network  of 
free  cementite,  a  structure  will  be  obtained  showing  both  sphe- 
roidal and  network  cementite. 


FIG.  92. — 1.40   per    cent.  Carbon  Steel,    Double    Quenched   from    1850°    and 

1375°  F,     X60. 

Spheroidal  Cementite. — The  importance  of  this  spheroidal- 
cementite  type  of  hyper-eutectoid  structure  as  a  wearing  surface 
cannot  be  over-emphasized.  When  such  a  steel  is  first  placed  in 
service,  the  tendency  will  be  for  the  martensite  gradually  to  wear 
away,  leaving  the  extremely  hard  spheroids  of  free  cementite  to  take 
the  wear.  We  then  have  the  ideal  conditions  for  a  wearing  or  bear- 
ing surface:  the  innumerable  "  points  "  of  cementite,  imbedded  in 
the  softer  and  tougher  martensite,  act  as  the  bearing-points;  and  this 
wearing  surface  may  be  ideally  lubricated  through  the  circulation 
of  oil  in  the  free  zone  representing  the  difference  between  the  surface 
of  the  cementite  and  that  of  the  martensite. 


164  STEEL  AND   ITS  HEAT  TREATMENT 

Spheroidal  Ferrite. — This  same  treatment  will  effect  the  "  spher- 
oidalizing  "  of  the  free  ferrite  of  the  hypo-eutectoid  zone  and  of  the 
core  in  a  like  manner.  In  Fig.  93  there  is  shown  the  core  of  a  case- 
hardened  steel  of  0.22  per  cent,  carbon  which  had  been  double 
quenched  at  1850°  and  1375°  F.  A  treatment  of  this  nature  will 
therefore  put  the  whole  steel,  both  case  and  core,  in  an  analogous 
condition,  and  the  effect  of  any  "  liquation  "  of  either  the  ferrite  or 
the  cementite,  caused  either  by  slow  cooling  or— with  cementite — 
by  oscillating  temperatures  of  carburization,  will,  for  the  most  part, 
be  overcome  by  such  treatment. 


FIG.  93. — Core  Containing  0.22  per  cent.  Carbon,  Double  Quenched  from  1850° 
F.  and  1375°  F.     (Bullens.) 

Avoiding  High  Quenching  Temperatures. — Referring  again  to  the 
effect  of  high  quenching  temperatures  upon  the  refinement  of  the 
core,  it  may  be  said  that  such  temperatures  will  necessarily  not  bring 
out  the  fullest  elimination  of  brittleness  in  the  core.  For  this,  as 
well  as  for  other  practical  reasons  involved  in  the  obtaining  of  such 
high  temperatures,  it  is  advisable  to  avoid  their  use.  This  may  be 
accomplished  in  two  ways:  by  avoiding  such  carburizing  methods 
as  will  necessitate  their  use,  as  will  be  evident  from  the  definition 
which  follows;  or  by  a  preliminary  quenching  directly  subsequent 
to  carburization  which  we  will  discuss  a  little  later. 


CASE   HARDENING:    THERMAL  TREATMENT  165 

Maximum  Efficiency  in  Case-hardened  Steels. — Gathering 
together  some  of  the  facts  previously  discussed,  we  will  state  that  the 
best  wearing  surface,  in  combination  with  minimum  brittleness  of  case 
(as  shown  by  the  absence  of  enfoliation)  and  of  core  (as  shown  by 
shock  tests),  as  well  as  with  minimum  difficulties  of  treatment,  will 
be  had  under  the  following  conditions : 

(1)  When  the  maximum  carbon  concentration  in  the  case  is 
greater  than  0.9  per  cent.,  but  does  not  exceed  that  amount  which  will 
cause  the  temperature  of  the  Acm  range  of  the  case  to  exceed  the 
temperature  of  the  A3  range  of  the  steel  of  the  core;  and  (2)  when 
the  following  conditions  subsequent  to  case  carburizing  are  rigorously 
observed  and  their  effect,  with  the  exception  of  a,  is  at  a  maximum: 
(a)  Slow  cooling  from  the  temperature  of  carburization  (which  we 
will  shortly  discuss);  (6)  quenching  from  a  temperature  slightly 
above  the  A3  range  of  the  initial  steel;  followed  by  (c),  a  quenching 
from  a  temperature  slightly  above  the  Ac  1.2. 3  range  of  the  steel. 

Maximum  Carbon  Content. — Although  the  first  statement 
regarding  the  maximum  carbon  content  which  we  have  recom- 
mended, that  is,  over  the  eutectoid  ratio,  is  in  direct  contradiction 
to  the  opinion  of  many  metallurgists,  it  is  nevertheless  strongly 
supported  by  industrial  results  as  well  as  by  theory.  But  it  should 
also  be  distinctly  noted  that  the  conditions  of  the  specific  heat 
treatments  under  the  second  statement  are  strongly  qualified,  in 
that  the  best  technical  methods — involving  accuracy  and  uniformity 
of  heating  and  heat  control — shall  be  instituted,  and  that  the  effect 
of  each  quenching  shall  be  at  a  maximum.  If  such  conditions  can- 
not be  complied  with,  it  will  be  decidedly  preferable  to  adopt  such 
methods  of  carburization  as  will  produce  a  maximum  carbon  content 
in  the  case  of  not  much  exceeding  0.9  per  cent. 

Maximum  Effect  for  Cementite  Solution. — For  reasons  which 
will  shortly  be  evident,  it  will  be  advisable  further  to  amplify  the 
proviso  that  "  the  effect  of  each  quenching  shall  be  at  a  maximum." 
First,  then,  in  regard  to  the  effect  of  the  initial  quenching  upon  the 
solution  of  the  cement ite.  It  is  well  known  that  the  solution  of  the 
free  cementite  in  hyper-eutectoid  steels  takes  place  very  slowly. 
Due  to  this  sluggish  action,  it  will  often  be  found  that  a  heating 
of  short  duration  slightly  above  the  Acm  range  will  not  entirely 
dissolve  the  excess  cementite.  And  further,  that  it  is  often  necessary, 
in  order  to  avoid  a  prolonged  heating  at  the  apparent  Acm  temper- 
ature, to  increase  this  temperature  to  a  considerable  extent.  Now 
when  the  maximum  carbon  content  of  the  case  is  such  that  the 


166  STEEL  AND  ITS  HEAT  TREATMENT 

theoretic  Acm  temperature  is  considerably  below  the  Ac3  range  of  the 
steel  of  the  core,  it  is  evident  that  there  will  be  little  difficulty  in 
satisfactorily  obtaining  the  full  solution  of  this  cementite.  But,  on 
the  other  hand,  if  the  two  temperatures  named  almost  coincide,  it  is 
manifest  that  the  maximum  effect  of  the  initial  heating  and  quench- 
ing relative  to  the  solution  of  the  free  cementite  will  not  always  be 
obtained  unless  such  heating  is  prolonged.  To  increase  the  duration 
of  this  heating  is  also  inadvisable,  because  this  would  tend  towards 
the  diffusion  or  equalization  of  the  carbon  content  in  the  various 
external  layers;  and  this,  in  turn,  would  be  contrary  to  the  purpose 
for  which  the  high  carbon  was  originally  obtained  through  carburiza- 
tion.  Again,  quenching  from  a  higher  temperature  than  that  origi- 
nally set  would  obviously  exceed  the  provisions  previously  named  as 
those  necessary  to  obtain  the  best  product,  and  for  the  present  may 
be  eliminated. 

Double  Initial  Quenching  for  Solution  of  Cementite. — It  will 
therefore  be  inadvisable  to  raise  the  maximum  carbon  content  of 
the  case  sufficiently  high  (through  carburization)  so  as  to  bring  about 
the  condition  of  affairs  which  we  have  just  been  discussing.  If  we 
abide  by  the  arbitrary  rules  which  we  have  laid  down,  the  only  way 
out  of  such  difficulty,  if  it  should  exist,  is  to  double  quench  from  the 
initial  temperature. 

Relation  of  Initial  Carbon  to  Maximum  Carbon. — Another 
variable  which  should  also  be  noted  under  this  subject  is  the 
influence  of  the  carbon  content  of  the  steel  of  the  core  upon  the  Ac3 
range.  A  study  of  the  chart,  Fig.  13,  will  show  that  between  the 
minimum  carbon  content  used  for  case-hardening  steels,  or  about 
0.05  per  cent.,  and  the  maximum  carbon,  or  about  0.25  per  cent., 
there  is  a  difference  of  about  125°  F.  This  will  mean  a  corresponding 
difference  in  the  possible  initial  quenching  temperature,  and  will,  in 
turn,  influence  the  factor  of  the  maximum  carbon  content  in  the  case 
which  it  is  possible  for  us  to  use  under  these  rules.  This  factor 
must  therefore  be  taken  into  account  in  the  method  of  carburizing. 
We  may  then  say  that  the  lower  the  carbon  content  of  the  steel 
to  be  carburized,  the  greater  may  be  the  maximum  carbon  content 
in  the  case — again  assuming  the  previous  conditions  to  hold. 

Double  Regenerative  Quenching. — Let  us  now  consider  the 
effect  of  the  initial  quenching  temperature  on  the  core.  In  the 
chapter  dealing  with  case  carburizing  it  was  stated  that  the  higher 
the  temperature  of  carburization,  and  the  greater  the  length  of  expo- 
sure at  that  temperature,  the  greater  would  be  the  grain  size  and  its 


CASE  HARDENING:    THERMAL  TREATMENT  167 

influence  upon  subsequent  regeneration.  Under  such  conditions 
it  will  not  always  be  possible  to  obtain,  by  a  single  initial  quenching, 
the  full  refinement  of  the  core.  The  only  alternative,  in  order  to 
satisfy  the  set  conditions,  will  be  to  double  quench  at  the  initial 
temperature. 

Slow  Cooling  after  Carburization. — In  a  previous  section  we  men- 
tioned that  the  first  step  subsequent  to  carburization  was  to  allow 
the  steel  to  cool  slowly  from  the  temperature  of  such  carburization. 
When  solid  cements  are  used,  the  method  involving  the  immediate 
removal  of  the  cemented  pieces  from  the  carburizing  boxes  and 
throwing  them  into  the  quenching  bath  cannot  be  too  strongly  con- 
demned, especially  if  there  is  to  be  no  regenerative  quenching. 
In  the  first  place,  it  is  a  practical  impossibility  to  remove  all  the 
pieces  from  the  box  and  to  so  quench  them  that  the  results  will  be 
identical.  This  statement  and  its  logical  conclusions  hardly  need 
further  explanation. 

In  the  second  place,  in  order  to  obtain  a  full  refinement  of  the 
steel,  it  is  absolutely  necessary  that  the  material  shall  be  reheated 
from  a  temperature  below  the  lowest  critical  range  to  a  temperature 
beyond  the  upper  critical  range,  for  otherwise  full  regeneration  will 
not  take  place.  If  the  objects  have  been  immediately  quenched 
from  a  temperature  near  that  of  the  carburization  (i.e.,  without  hav- 
ing been  previously  slow-cooled),  the  grain  size  retained  by  this 
quenching  will  be  that  characteristic  of  the  highest  temperature 
reached  during  the  carburization.  The  grain  size  thus  given  to  the 
core  will  be  large,  because  the  temperature  of  carburization  must 
obviously  be  high  if  quenching  is  to  take  place  before  the  temperature 
of  the  steel,  during  removal  from  the  box,  falls  below  that  of  the 
hardening  point.  If  the  steel  should  be  put  into  service  in  the  con- 
dition just  mentioned,  it  would  not  be  capable  of  withstanding  any 
great  amount  of  shock  on  account  of  its  inherent  brittleness.  And 
even  if  the  first  haphazard  quenching  should  be  followed  by  a  re- 
heating and  quenching  from  slightly  above  the  lowest  critical  range 
it  is  evident  from  previous  discussion  that  the  steel  of  the  core  as  a 
whole  will  not  be  regenerated. 

In  other  words,  if  the  carburization  has  given  the  proper  maximum 
carbon  content  in  the  case,  previously  stated,  such  a  quenching  will 
be  of  little  economic  importance  because  it  must  always  be  followed 
by  the  double  quenching  (regenerative  and  hardening)  necessary  to 
produce  maximum  efficiency.  Under  such  conditions,  and  for  both 
theoretic  and  practical  reasons,  it  is  advisable  to  permit  the  car- 


168  STEEL  AND   ITS  HEAT  TREATMENT 

burized  steel  to  cool  in  the  boxes  to  a  temperature  at  least  lower  than 
that  of  the  Arl  range. 

Benefits  from  Preliminary  Quenching. — Leaving  aside  the 
consideration  of  those  steels  which  require  only  a  surface  hardness, 
there  are  only  two  benefits  which  can  accrue  from  quenching  directly 
after  carburization.  First,  there  is  the  prevention  of  the  "  liquation  " 
of  the  excess  cementite  during  slow  cooling,  with  the  possible  resulting 
disadvantages  through  enfoliation,  or  similarly,  the  liquation  of  the 
ferrite.  The  author  believes  that  the  effect  of  this  phenomenon  of 
liquation,  although  strongly  emphasized  by  Giolitti,  may  be  largely 
counteracted  by  the  results  of  the  effective  double  quenching  and  its 
consequent  "  spheroidalizing  "  action.  The  use  of  the  preliminary 
quenching,  assuming  the  proper  maximum  carbon,  may  be  regarded 
as  of  indirect  benefit  in  this  first  proposition. 

Second,  and  of  particular  and  direct  importance,  is  when  the 
maximum  carbon  content  of  the  case  exceeds  that  amount  at  which 
the  temperature  of  the  Acm  range  is  equal  to,  or  greater  than,  the 
temperature  of  the  Ac3  range  of  the  steel  of  the  core.  Under  these 
conditions  the  preliminary  quenching — as  we  call  it — will  prevent 
the  precipitation  and  coagulation  of  the  excess  cementite  into  the 
network  and  spines  which  are  so  difficult  to  redissolve  during  regen- 
erative heating.  Consequently,  this  preliminary  quenching  will 
permit  the  direct  use  of  the  regenerative  quenching  at  its  proper 
temperature,  even  though  the  carbon  content  of  the  case  is  higher 
than  the  governing  ratio  between  Acm  and  Ac3  and  which,  under 
conditions  of  slow  cooling,  would  demand  the  use  of  a  higher  regenera- 
tive quenching.  It  is  manifest,  however,  that  such  preliminary 
quenching,  to  be  effective,  must  take  place  at  a  temperature  higher 
than  the  specific  Acm  temperature,  or  at  about  that  of  the  cementa- 
tion proper. 

Use  of  Salt-bath  Heating. — Before  summing  up  the  treatments 
given  in  the  foregoing  pages,  there  are  three  points  of  practical  inter- 
est which  should  be  noted.  The  first  of  these  has  to  do  with  the 
method  of  heating  the  steel  for  quenching.  It  is  obvious  that  oxida- 
tion, even  of  very  slight  amount,  must  be  entirely  prevented.  The 
best  and  surest  method  of  attaining  this  is  by  the  use  of  molten  baths. 
Of  these,  the  salt  baths  are  to  be  preferred  to  the  use  of  lead,  at  least 
for  temperatures  over  1500°  F.,  on  account  of  the  poisonous  fumes  of 
the  latter  at  the  high  temperatures. 

Interrupted  Regenerative  Quenching. — The  second  item  refers 
to  the  regenerative  quenching.  On  account  of  the  tendency  of  the 


CASE  HARDENING:    THERMAL  TREATMENT  169 

high-carbon  steels  to  check  or  crack  when  high  quenching  tempera- 
tures are  used,  it  is  advisable  to  remove  the  steel  from  the  water 
bath  when  its  red  color  is  seen  to  disappear.  As  the  steel  "  loses  its 
color  "  at  a  temperature  under  that  of  the  lowest  critical  range- 
that  is,  below  that  temperature  at  which  the  transformation  in  cool- 
ing is  totally  effected,  it  is  evident  that  this  interrupted  cooling  will 
in  no  wise  affect  the  regeneration  of  the  core.  Its  influence  upon  the 
structure  of  the  case  will  also  have  little  practical  importance, 
primarily  because  it  is  not  desired  through  this  quenching  to  obtain 
a  maximum  hardness;  and  further,  because  there  will  be  little  or  no 
tendency  for  any  excess  cementite  to  precipitate  as  a  network  struc- 
ture. If  any  of  the  excess  cementite  should  be  thrown  out  of  solu- 
tion, it  is  more  apt  to  be  of  the  spheroidal  type.  Whether  or  not  this 
cooling  is  interrupted  at  about  900°  F.,  it  is  always  advisable  to 
remove  the  steel  from  the  bath  before  it  has  become  entirely  cold. 

Coagulation  of  Cementite. — In  the  third  place,  we  would  refer 
briefly  to  the  "  hardening  "  or  second  quenching.  If  the  case  con- 
tains greater  than  the  eutectoid  ratio  of  carbon,  the  duration  of  the 
heating  at  this  lower  temperature  should  not  be  prolonged  over  a 
greater  period  than  is  necessary  thoroughly  and  uniformly  to  heat 
the  case  to  the  proper  hardening  temperature.  A  prolonged  heating 
would  have  the  tendency  to  coagulate  the  cementite  which  is  ordi- 
narily precipitated  at  this  temperature,  thus  opposing  the  realiza- 
tion of  the  conditions  of  maximum  effectiveness. 

Summary. — We  may  sum  up  the  general  situation,  and  give  to 
each  class  of  steel  the  treatment  which  we  recommend  to  obtain  the 
"  best  wearing  surface,"  combined  with  minimum  brittleness  of  case 
and  core. 

Classification  of  Case-carburized  Steels 

Group  A.  Steels    case    carburized   at   temperatures   approximating 

that  of  the  upper  critical  range  of  the  initial  steel. 
Group  B:  Steels  case  carburized  at  temperatures  considerably  exceed- 
ing that  of  the  upper  critical  range  of  the  initial  steel. 
Class  1.  Maximum  carbon  content  of  the  case  does  not  exceed 

0.9  per  cent. 

Class  2.  Maximum  carbon  content  of  the  case  greater  than 
0.9  per  cent.,  but  is  less  than  when  Acm  of  the  case 
equals  Ac3  of  the  core. 

Class  3.  Maximum  carbon  content  of  the  case  greater  than 
that  specified  under  (2). 


170  STEEL  AND  ITS  HEAT  TREATMENT 

Classification  of  Treatments  for  Specific  Steels 

Group  A.        Class  1.        Treatment  I. 

2.  II. 

3.  III.  or  IV. 

Group  B.        Class  1.  Treatment  II. 

2.  II. 

3.  III.  or  IV. 
Treatment  I. 

a.  Cool  slowly. 

b.  Quench  from  slightly  over  the  Acl.2.3,  or  about  1375°  F. 

Treatment  II. 

a.  Cool  slowly. 

b.  Quench  from  slightly  over  Ac3  of  the  core.     Dependent  upon 

the  carbon  content,  this  will  vary  from  1650°  to  about 
1525°  F. 

c.  Quench  from  slightly  over  Acl.2.3,  or  about  1375°  F. 

Treatment  III. 

a.  Quench   directly  subsequent   to  carburization,  without  slow 

cooling,  from  at  or  near  the  temperature  of  carburization 
but  not  lower  than  Acm.  Dependent  upon  the  carbon 
content  of  the  case,  Acm  will  vary  from  about  1650°  F.  for 
1.20  per  cent,  carbon  (or  thereabouts),  to  about  1800°  for 
1.45  per  cent,  carbon.  It  is  not  advisable  to  quench  at  a 
temperature  higher  than  1800°  F. 

b.  Treatment  as  in  II,  6  and  c. 

Treatment  IV. 
a.  Cool  slowly. 

6.  Quench  from  a  temperature  over  the  Acm,  dependent  upon  the 
carbon  content  of  the  case.     (See  III,  a.) 

c.  Quench  from  slightly  over  Acl.2.3,  or  about  1375°  F. 

NOTE  :  This  treatment  requires  a  slight  sacrifice  in  the  minimum 
brittleness  of  core  in  order  to  obtain  "  best  wearing  surface." 

Mechanical  Effects  of  Treatments. — The  effect  upon  the  mechan- 
ical properties  of  the  case  and  core  of  various  treatments  is  given  in 
the  following  table  taken  from  Guillet.  The  steel  used  was  of  the 
ordinary  type  for  case  hardening,  classed  as  "  extra  soft." 


CASE  HARDENING:    THERMAL  TREATMENT 


171 


Treatment. 

Resistance  of 
the  Core  to 
Shock  in  kg.m. 

Surface  Hard- 
ness of  the    , 
Case,  Shore 
Method. 

Non-cemented  steel,  heated  at  1700°  F 
in  air      

.  and  cooled 

20.6 

Non-cemented  steel,  quenched  at  1700C 
Steel  cemented  at  1830°  F.  for  0.047  in 
slowly    .  .           

F.  in  water 
.  and  cooled 

23.8 
13.5 

38.5 

Same  cementation;  quenched  at  1830° 
Same  cementation;    quenched  twice  i 
1830°  and  1375°  F  

F.  in  water  . 
n  water,  at 

23.2 
25.5 

79.8 
84.0 

Further  Treatments  not  giving  Maximum  Efficiency. — Case- 
hardened  objects  having  a  comparatively  thin  cemented  zone 
(yg-  in.  or  less)  may  broadly  be  divided  into  those  articles  which 
require  only  surface  hardness  and  work  under  fairly  uniform  pressure 
without  shock,  and  those  articles  which  must  withstand  shock, 
bending  strains,  etc.  We  have  discussed  at  some  length  both  the 
carburization  and  the  heat  treatment  which  are  required  by  those 
of  the  latter  class.  The  heat  treatment  of  those  articles  of  the  first 
class  we  have  previously  referred  to,  but  for  purposes  of  summarizing 
we  may  divide  it  as  follows; 

Treatment  V. 

a.  Quench  directly  after  carburization  (without  slow  cooling), 
but  at  a  temperature  not  less  than  1350°  F.,  or  that  of  Arl. 
The  results  may  be  varied  over  a  wide  range  according  to  the 
temperature  of  quenching. 

NOTE:  This  treatment  is  for  those  articles  which  merely  demand 
a  hard  surface,  and  in  which  brittleness  and  enfoliation  may  not  be 
considered. 


Treatment  VI. 

a.  Cool  slowly. 

b.  Quench  from  slightly  over  Ac3  of  the  initial  steel,  varying  from 

1650°  to  1525°  according  to  the  carbon. 

NOTE:  This  treatment  is  for  those  articles  which  demand  a 
tough  core  and  a  comparatively  hard  surface — that  is,  the  elimination 
of  brittleness  in  the  core  is  of  more  importance  than  maximum 
surface  hardness. 


172  STEEL  AND   ITS  HEAT  TREATMENT 

Treatment  VII.     (Similar  to  Treatment  I.) 

a.  Cool  slowly. 

b.  Quench  from  about  1375°  F.,  or  slightly  over  Acl. 

NOTE  :  This  treatment  is  for  articles  which  demand  a  maximum 
surface  hardness,  or  as  much  as  can  be  obtained  from  a  single  quench- 
ing, without  reference  to  the  brittleness  of  core  or  to  the  dangers 
of  enfoliation  through  the  presence  of  free  cementite.  With  low 
temperatures  of  carburization  and  with  a  carbon  maximum  of  0.9 
per  cent,  this  classification  would  of  course  correspond  to  Group  A, 
Class  1. 

Alloy  Steels. — The  treatment  of  alloy  steels  will  be  considered 
under  their  respective  chapters.  In  the  main,  however,  the  theory 
of  treatment  does  not  vary,  although  the  actual  temperatures  may 
be  changed  on  account  of  the  influence  of  certain  alloys  upon  the 
position  of  the  critical  ranges. 


CHAPTER  VIII 
HEAT   GENERATION 

Distinctive  Conditions. — In  industrial  heating,  and  particularly 
the  sequence  of  operations  applied  to  the  heat  treatment  of  steel, 
it  is  but  hard  common  sense  to  state  that  there  is  no  general  solution 
applicable  to  the  heating  element  or  furnace.  The  application  of 
heat  to  these  various  operations,  with  the  accompanying  design  of 
furnace  equipment,  is  an  engineering  problem,  and  it  must  be  con- 
sidered as  such,  and  in  the  broadest  manner,  if  the  greatest  efficiency 
is  to  be  obtained.  No  single  type  of  furnace,  fuel  nor  "  system  " 
of  burning  can  be  applied  as  a  "  cure-all."  Each  case  must  be  dealt 
with  on  its  merits  and  the  furnace  and  the  fuel  and  their  application 
to  the  work  in  hand  must,  in  the  final  analysis,  be  based  upon  the 
results  obtained,  measured  commercially.  Consequently,  as  no  two 
problems  are  exactly  alike,  it  necessarily  follows  that  the  furnace 
equipment  must  be  designed  to  suit  the  individual  plant  with  its 
distinctive  conditions.  The  average  heat-treatment  shop  needs  a 
drastic  awakening  from  the  lethargy  of  "  cut-and-dried  "  systems, 
poorly  designed  and  "  home-made  "  furnaces,  inefficient  treatment 
and  handling  of  products. 

Quality  of  Product  vs.  Cost. — Quality  of  product  and  cost  of 
manufacture  are  the  basis  of  heat-treating  operations.  Quality  of 
product  covers  the  proper  heating  of  the  material  to  meet  the  met- 
allurgical requirements,  and  its  physical  condition  to  meet  the 
mechanical  requirements.  Cost  of  manufacture  includes  the  cost 
of  fuel,  power,  labor,  special  equipment  and  material,  such  as  boxes, 
tools,  quenching  fluids,  etc.,  as  well  as  fixed  charges  on  the  equip- 
ment, floor  space,  etc.  Many  of  the  mistakes  that  have  been  made  in 
heat-treatment  installations  are  due  to  the  fact  that  the  problem  has 
been  considered  from  the  standpoint  of  fuel  alone  or  of  the  first  cost 
of  installation.  Such  a  view  is  short-sighted,  for  the  cost  of  fuel 
alone  makes  up  but  a  comparatively  small  part  of  the  total  produc- 
tion cost.  But  when  these  items  are  considered  in  their  proper  place 
with  the  other  items  of  operating  cost  and  with  the  proper  inter- 

173 


174  STEEL  AND   ITS  HEAT  TREATMENT 

pretation  of  the  relation  of  these  to  the  cost  of  the  finished  product, 
it  will  generally  be  found  that  the  cost  of  fuel  and  the  cost  of  installa- 
tion become  of  secondary  importance  in  measuring  or  setting  a  stand- 
ard of  excellence  to  which  the  product  must  conform.  In  other 
words,  the  ultimate  aim  of  any  heat-treating  process,  from  the 
economic  standpoint,  is  to  obtain  the  best  heating  of  the  product  at 
the  least  total  cost. 

The  Standard  Heating  Unit. — There  is  no  definite  standard 
employed  for  the  measurement  of  production  cost  in  industrial  heat- 
ing, as  with  power  or  light,  because  the  conditions  are  continually 
varying  and  there  is  no  one  definite  point  or  method  to  determine 
the  cost.  In  power  the  test  is  the  cost  per  brake  horse-power  hour 
at  the  shaft  of  the  machine,  irrespective  of  the  purpose  of  the  appara- 
tus. In  light  the  test  is  the  cost  per  candle  power  hour,  irrespective 
of  the  fuel  employed  or  the  means  of  utilizing  or  applying  it.  In 
electricity  it  is  the  kilowatt  hour  measured  at  some  definite  point. 
The  nearest  approach  we  can  make  to  a  standard  for  the  commercial 
determination  in  industrial  heating  is  to  suggest — "  the  cost  per  unit 
of  quantity  of  given  quality."  This  is  somewhat  indefinite  and  dif- 
ficult of  location,  owing  to  the  many  different  standards  for  quality 
and  the  great  latitude  in  furnace  design  which  affects  the  elements 
entering  into  the  cost  of  production  above  noted. 

Such  a  standard  means  the  abandonment  of  the  technique  of 
combustion  and  other  thermal  considerations  that  are  usually  fol- 
lowed. It  means  that  the  cost  of  finished  product  is  paramount, 
regardless  of  fuel  cost.  And  it  makes  a  point  of  considering  first 
of  all  the  application  of  heat  to  the  stock,  and  then  an  efficient 
method  of  handling  that  material.  Working  along  these  lines  has 
produced  real  results  in  heating  efficiency  (if  such  a  term  is  per- 
missible) with  oil  fuel;  the  gas  fraternity  are  beginning  to  recognize 
the  basic  truth  of  the  statement  that  fuel  cost  does  not  determine 
heating  cost;  and  the  electrical  men  are  slowly  falling  in  line.  Each 
fuel  has  limits  within  which  it  can  be  used,  and  these  are  determined 
by  the  nature  of  operations  regardless  of  fuel  cost. 

Heating. — Any  talk  upon  industrial  heating  must  necessarily 
take  into  account  the  right  fuel  and  its  proper  application,  a  suitable 
furnace  design  and  construction,  and  a  proper  layout  and  efficient 
handling  of  materials.  Although  each  of  these  propositions  is,  in 
a  sense,  distinct,  it  is  obvious  that  each  involves  and  must  be 
co-ordinated  with  the  others.  Similarly,  the  broad  subject  of 
heating  must  deal  with: 


HEAT  GENERATION  175 

(1)  The  generation  of  the  heat — the  fuel; 

(2)  A  system  for  applying  the  heat — the  furnace; 

(3)  The  utilization  of  the  heat — the  uniform  heating  of  the 
stock; 

(4)  The  conservation  of  the  heat — guarding  against  losses. 
Comparative  Fuel  Costs. — The  comparison  of  initial  fuel  costs 

is  always  an  interesting  subject,  but  unless  it  is  carefully  amplified 
and  taken  only  in  "  small  doses,"  it  is  apt  to  prove  the  truth  of  the 
old  saying  that  "  a  little  learning  is  a  dangerous  thing."  All  of  the 
factors  given  in  the  previous  paragraph  must  be  considered  in  the 
selection  of  any  fuel,  for,  after  all  is  said  and  done,  it  is  the  cost 
of  heating  as  shown  by  the  finished  product,  and  not  the  B.T.U.  cost 
of  fuel,  which  counts  the  most. 

The  chart  given  on  page  176  illustrates  graphically  the  relation- 
ship between  commercial  fuels,  based  on  their  heat  unit  cost. 

This  chart  facilitates  the  determination  of  the  cost  per  million 
B.T.U. 's  of  commercial  fuels  at  different  prices;  the  relative  prices 
for  different  fuels  at  a  definite  price  for  one  fuel  or  per  million  B.T.U. 's 
and  so  forth. 

To  illustrate — the  cost  per  million  B.T.U.'s  would  be:  at  3  cents 
per  gallon  for  fuel  oil— 21.5  cents;  at  20  cents  per  M  for  1000 
B.T.U.  natural  gas— 20  cents;  and  at  $5.00  per  ton  for  12,000  B.T.U. 
coal — 20.8  cents. 

Again,  at  $5.00  per  ton  for  12,000  B.T.U.  coal,  the  relative  prices 
for  the  other  fuels,  keeping  the  same  B.T.U.  cost,  would  be — $5.80 
per  ton  for  14,000  B.T.U.  coal;  21  cents  per  M  for  1000  B.T.U. 
natural  gas;  12.6  cents  per  M  for  city  gas;  2.94  cents  per  gal. 
for  oil;  2.6  cents  per  M  for  125  B.T.U.  producer  gas,  etc. 

Further — at  an  assumed  cost  of  30  cents  per  million  B.T.U.'s, 
the  relative  prices  for  various  fuels  would  be — 4.2  cents  per  gal.  for 
oil;  5  cents  per  M  for  165  B.T.U.  producer  gas;  9  cents  per  M  for 
water  gas;  18  cents  per  M  for  city  gas;  $6.00  per  ton  for  10,000 
B.T.U.  coal,  etc. 

The  B.T.U.  Value. — The  fanacy  of  selecting  a  fuel  merely  by 
its  B.T.U.  value  alone,  however,  should  be  self-evident.  To  illus- 
trate, take  the  case  of  anthracite  coal:  when  broken  into  pieces  the 
size  of  a  man's  fist  it  is  used  in  the  ordinary  hot-air  "  heater  "  or 
house  furnace;  when  crushed  to  a  smaller  size  it  is  used  in  the  kitchen 
stove;  crushed  to  rice-size  it  may  be  used  for  forced-draft  boilers; 
pulverize  it  to  a  dust  and  it  may  be  used  in  the  "  powdered  coal  " 
systems.  But  would  the  furnace  size  be  suitable  for  the  kitchen 


176 


STEEL   AND   ITS    HEAT   TREATMENT 


S         8         8         S 
^sqiooog 


'HO  jo  'I^O  J9d  Jo  SB  o  jo  "jj  -no  0001  aed  sixtiao  ut  aou^ 


HEAT  GENERATION  177 

stove,  or  would  stove  coal  satisfy  the  conditions  necessary  for  the 
latter  systems?  Assuredly  not;  and  yet  the  B.T.U.  value  of  the 
coal  remains  absolutely  unchanged.  In  other  words,  it  is  not  the 
cost  of  the  B.T.U. 's  in  coal,  but  all  in  the  manner  of  its  us.e. 

The  Combustible  Mixture. — Again,  the  so-called  B.T.U.  com- 
parison of  fuels,  which  has  long  been  generally  accepted  as  standard, 
is  misleading.  In  fact,  it  is  not  the  B.T.U.  value  of  the  fuel,  but 
the  B.T.U.  value  of  the  combustible  mixture  that  counts.  There  is 
not  as  much  difference  in  the  heating  value  of  the  combustible  mix- 
tures of  the  various  gases  as  there  is  between  the  heating  value  of 
the  gases  themselves  without  considering  the  combustible  mixture. 
For  example,  if  city  gas  were  assumed  to  be  600  B.T.U.  and  producer 
gas  120  B.T.U.,  the  heating  value  of  the  gases  would  generally  be 
considered  in  the  proportion  of  five  to  one;  and  many  comparisons 
are  made  upon  this  basis.  In  practice,  however,  whether  it  be  in 
an  internal  combustion  engine  or  in  a  furnace,  the  actual  values  of 
the  fuels  are  not  anywhere  near  the  ratio  of  five  to  one;  if  they  were, 
then  an  engine  of  given  size  cylinder  which  would  develop  100  H.P. 
with  city  gas  would  only  give  20  H.P.  with  producer  gas.  This 
comparison  most  people  would  agree  is  ridiculous  and  unreasonable 
on  its  face,  in  the  light  of  practice  in  power;  and  yet  the  same  people 
do  not  hesitate  to  draw  such  a  comparison  when  considering  heat, 
although  the  determinative  conditions  are  just  as  true  in  one  case  as 
in  another. 

The  Right  Fuel. — The  generation  of  heat  for  heat-treatment 
purposes  involves  the  proper  application  of  the  right  fuel.  Super- 
ficially the  choice  of  fuel  appears  to  be  a  simple  one.  Too  many 
persons,  however,  are  prone  to  select  off-hand  some  fuel  such  as 
coal  because  the  initial  cost  is  low,  or  maybe  natural  gas  if  it  is  near 
at  hand,  or  even  electricity  just  because  it  sounds  attractive  and  is 
easily  controlled.  Although  each  of  these  in  its  proper  sphere  would 
be  the  logical  source  of  heat,  as  general  proposition  no  one  fuel  is 
appropriate  to  every  case.  Neither  low  initial  cost,  nor  local  supply, 
nor  ready  control  sums  up  the  situation.  These  are  but  factors  in  the 
case  as  a  whole,  and  the  value  of  one  fuel  cannot  be  measured  by 
its  use  in  the  way  another  fuel  would  be  used.  There  is  always  a 
right  fuel  for  the  particular  work  in  hand,  so  that  each  problem 
must  be  thoroughly  studied  and  understood  if  the  best  solution  is 
to  be  had. 

Fuel  Efficiency. — There  is,  or  at  least  should  be,  no  argument 
on  the  fuel  question.  The  relationship  of  the  various  fuels  is  fixed 


178  STEEL  AND   ITS   HEAT  TREATMENT 

by  physical  law  and  commercial  conditions.  The  term  "  fuel 
efficiency  "  is  not  used  in  power  nor  in  domestic  house-heating  work; 
and  neither  does  it  exist  in  industrial  heating.  Generally  speaking, 
it  may  be  assumed  that  any  difference  in  results  between  oil,  or  gas, 
or  electricity  for  heating  furnaces  should  be  attributed  to  the  manner 
in  which  the  heat  is  applied  to  the  stock  and  not  to  any  inherent 
advantage  in  one  form  of  energy  over  the  other.  It  is  not  proper  to 
say  that  oil  is  cheaper  than  electricity  or  coal  or  gas,  or  vice  versa, 
or  that  a  higher-price  fuel  can  displace  another  because  the  former 
is  "  more  efficient."  There  is  no  such  thing  as  that  one  will  do  more 
than  another.  It  is  the  nature  of  the  operation  and  the  manner  of 
applying  heat  that  counts,  and  not  the  fuel.  Thus,  the  manner  of 
its  use  may  be  efficient  or  the  means  of  employing  or  utilizing  it, 
but  certainly  not  the  fuel  itself.  It  can  be  said  that  electricity  in  a 
given  type  of  furnace  will  produce  a  better  result  than  oil  in  a  given 
type  of  furnace,  but  it  will  be  noticed  in  so  doing  that  it  is  the  dif- 
ference in  the  manner  of  applying  the  heat,  which  is  equivalent  to  a 
difference  in  furnace  design,  that  determines  the  result,  and  not  to 
any  advantage  in  one  form  of  energy  over  another.  The  term  "  fuel 
efficiency/'  therefore,  is  misleading  in  that  it  is  employed  to  express 
a  condition  that  does  not  exist. 

Fuel  vs.  Operations. — It  may  be  said  that  the  extent  of  the 
heating  operation  more  or  less  determines  the  fuel.  It  would  be  good 
practice  to  employ  city  gas  for  the  annealing  of  a  small  quantity  of 
wire  in  a  loft  building ;  and  with  a  larger  quantity  and  other  working 
conditions  the  fuel  might  be  oil;  but  if  the  operation  be  conducted 
on  a  still  larger  scale,  as  in  a  rolling  mill  where  the  size  and  type 
of  furnace  will  so  peimit,  then  the  fuel  might  be  producer  gas  or  coal. 
Thus,  city  gas  at  $1.00  a  thousand  for  many  operations  might  be 
recommended  in  preference  to  oil  at  1  cent  a  gallon,  and  electricity 
at  any  price  in  preference  to  oil  or  gas  at  any  price.  In  annealing, 
for  instance,  we  might  use  oil  for  certain  results  and  in  same  room  use 
coal  for  annealing  the  same  metal,  but  for  different  results.  Thus 
the  nature  of  the  operation  more  or  less  determines  the  working 
limits,  regardless  of  fuel  cost. 

The  "  Fluid  Fuel." — We  can  even  go  a  step  further  and  state 
that  no  one  fuel  has  a  monopoly  on  uniform  heating,  or  control,  or 
economy.  At  the  present  time  there  are  many  furnaces  of  good 
design,  operating  on  cheap  coal,  which  produce  a  better  quality  of 
product  at  less  cost  and  maintain  more  uniform  temperatures, 
with  more  accurate  control,  than  other  furnaces  of  poor  design  built 


HEAT  GENERATION  179 

for  the  same  purpose,  using  either  oil  or  clean- washed  producer  gas. 
All  other  things  being  equal,  a  "  fluid  fuel  "  (as  distinguished  from 
a  solid  fuel)  is  generally  to  be  preferred,  as  it  lends  itself  more  readily 
to  accurate  control.  Ordinarily  it  would  be  assumed  that  a  fluid 
fuel  would  permit  of  greater  flexibility  of  operation  than  a  solid  fuel, 
and  it  invariably  will  when  all  other  conditions  are  equal.  However, 
as  previously  noted,  there  are  many  cases  where  the  advantages  of 
the  more  flexible  fuel  are  lost  with  inefficient  means  of  utilizing  it. 
But  it  is  misleading  to  couple  any  fuel  with  the  term  Uniform 
Heating,  or  Control,  or  Economy,  without  specifying  or  qualifying 
the  manner  in  which  it  is  to  be  used.  It  is  the  manner  of  applying 
the  heat  and  not  the  fuel  which  thus  determines  the  success  or  failure 
of  the  operation. 

Selection  of  Fluid  Fuel. — Even  the  selection  of  a  fluid  fuel  is 
dependent  upon  conditions  other  than  heating  value  or  composition 
of  the  fuel  itself.  For  instance,  we  might  consider  a  very  small 
furnace  for  annealing,  tempering  or  hardening  small  pieces  of  stock. 
There  would  seem  to  be  no  question  but  that  gas  would  be  the  fuel 
most  generally  preferred.  But  even  the  selection  of  the  gas  itself 
would  be  dependent  upon  conditions  other  than  that  of  temperature 
control.  To  illustrate:  while  one  might  use  either  producer  gas, 
water  gas,  city  gas  or  natural  gas  in  the  case  just  mentioned,  it 
would  not  follow  that  each  of  the  fuels  could  be  considered  if  the 
operation  was  one  requiring  a  very  high  temperature,  such  as  welding. 
The  same  conditions  would  hold  good  if  the  operation  was  one  requir- 
ing a  very  low  temperature,  such  as  japanning.  If,  however,  the 
operation  were  to  be  conducted  on  a  large  scale,  and  one  involving 
the  use  of  a  large  furnace  in  which  there  would  be  ample  room  for 
combustion,  there  would  be  brought  into  competition  with  these 
gases  a  liquid  fuel  in  the  form  of  oil,  which,  by  reason  of  the  latitude 
afforded  in  furnace  design,  could  compete  from  the  standpoint  of 
temperature  control;  and,  depending  upon  the  market,  it  might  com- 
pete in  the  matter  of  price. 

Influence  of  Working  Conditions. — There  is  generally  a  confusion 
between  the  terms  Uniform  Heat  Distribution  and  Uniform  Fuel 
Distribution;  the  two  do  not  necessarily  go  together,  although  at 
times  they  do.  For  instance,  in  the  case  of  japanning,  requiring  a 
low  temperature,  particularly  in  small  and  medium-size  ovens,  a  fluid 
fuel  in  the  form  of  gas  is  generally  preferred.  The  reason  for  this 
is  that  the  fuel  may  be  applied  on  all  sides  of  the  oven  and  burned  in 
small  quantities  at  different  points.  This  could  not  be  as  readily 


180  STEEL  AND   ITS  HEAT  TREATMENT 

done  with  a  liquid  fuel  like  oil,  which  by  reason  of  its  great  calorific 
power  and  lack  of  control  when  burned  in  very  small  quantities  in  a 
very  limited  space,  would  prohibit  a  distribution  of  the  fuel  in  the 
manner  usually  provided  for  gas.  If  the  even  were  large  and  there 
was  plenty  of  room  for  combustion  and  distribution  of  heat  through 
flues,  it  would  be  possible  to  approach  the  same  conditions  of  uniform 
heating,  as  far  as  the  stock  is  concerned,  without  uniform  distribution 
of  fuel.  In  one  case  the  result  of  heating  the  material  is  accomplished 
by  distributing  the  points  of  heat  generation,  while  in  the  other  it  is 
produced  by  localizing  the  point  of  heat  generation  and  distributing 
the  heat  after  it  is  generated.  This  goes  to  show  why  working  con- 
ditions are,  as  a  rule,  determinative  not  only  of  the  fuel,  but  of  the 
manner  in  which  it  may  be  employed.  Also,  to  show  that  the  con- 
clusions formed  in  one  case  may  be  reversed  in  another,  when  a  change 
in  working  conditions  makes  it  either  possible  or  desirable,  even 
though  there  be  no  change  in  the  composition  or  price  of  the  fuels 
themselves. 

Regeneration. — The  question  of  regeneration  or  recuperation 
is  usually  misunderstood.  Recuperation  is  generally  commercially 
desirable  on  general  principles,  but  there  are  times  when  it  is  phy- 
sically necessary,  irrespective  of  the  economy  of  the  operation.  For 
instance,  neither  hot  nor  cold  producer  gas  is  suitable  for  high  forging 
or  welding  heats  in  furnaces  without  regeneration  or  recuperation, 
because  the  heating  value  of  the  fuel  is  so  low.  Because  of  their 
greater  heat  value,  water  gas,  natural  gas,  city  gas  or  fuel  oil  might 
be  used  for  operations  with  which  producer  gas  could  not  be  con- 
sidered, for  the  reasons  given  above.  But  if  the  furnaces  were  large 
and  the  principles  of  recuperation  could  be  employed,  then  the 
producer  gas  could  compete,  and  the  extent  to  which  it  could  com- 
pete would  be  determined  by  fuel  cost  coupled  with  the  manner  in 
which  the  fuels  were  employed.  Very  often  a  comparison  is  made 
with  producer  gas  used  in  this  form  against  the  other  fuels  to  show 
that  the  gas  would  be  the  cheaper.  But  this  in  itself  is  not  conclusive 
unless  the  determination  is  based  upon  the  results  secured  by  the 
use  of  the  other  fuels  in  a  manner  which  would  involve  recuperation, 
and  in  this  way  take  advantage  of  every  possible  saving.  Producer 
gas,  as  a  rule,  would  show  the  highest  efficiency  from  the  standpoint 
of  recuperation,  because  the  volume  of  the  inert  or  non-combustible 
gases  is  greater;  but  it  does  not  follow  that  this  efficiency  of  recupera- 
tion is  in  itself  determinative  of  the  fuel. 

Cost  of  Delivering  Fluid  Fuels. — Even  though  fuels  were  com- 


HEAT  GENERATION  181 

pared  in  the  manner  above  indicated,  it  would  not  follow  that  the 
result  would  be  as  conclusive  from  the  standpoint  of  industrial  heating 
engineering  as  it  would  be  from  that  of  fuel.  The  reason  for  this  is 
that  some  fuels  by  their  very  nature  are  better  adapted  to  the 
manipulation  of  conditions  governing  a  manufacturing  process 
than  others.  For  instance,  fuel  oil,  or  any  cold-washed  gas  that 
could  be  delivered  to  a  furnace  through  pipes,  and  thus  distributed 
to  scattered  units  throughout  a  plant,  could  be  more  easily  handled 
than  producer  gas  delivered  in  a  hot  state  through  a  flue. 

Cost  Factors  with  Producer  Gas.— The  conditions  that  hold  good 
with  an  open-hearth  furnace  do  not  obtain  in  a  small  forging  or  heat- 
treating  installation  made  up  of  scattered  units.  There  is  no  question 
but  that  the  B.T.U.  cost  at  the  producer  is  less  with  hot  gas  than  with 
cold  washed  gas,  because  of  the  heat  loss  in  the  latter  in  the  processes 
of  scrubbing,  drying  and  cleaning.  On  this  comparison  of  B.T.U. 
cost  at  the  producer  much  has  been  written  and  many  claims  have 
been  made  for  superiority  one  way  and  another.  It  must  be  borne 
in  mind,  however,  that  in  industrial  heating  it  is  not  the  cost  at  the 
producer  or  at  the  point  of  fuel  supply  that  counts,  but  rather  the  cost 
of  the  finished  product  at  the  delivery  end  of  the  furnace.  This 
heating  cost,  as  it  might  be  termed,  takes  into  account  not  only  the 
fuel  at  the  furnace,  but  the  utilization  of  the  fuel  in  the  furnace;  and 
on  top  of  this,  the  labor  and  any  other  charges,  such  as  interest  and 
depreciation,  that  may  be  legitimately  charged  against  the  equip- 
ment itself.  The  fuel,  at  best,  is  secondary  and  not  of  the  para- 
mount importance  which  many  have  erroneously  tried  to  make  it. 

Fuel  Equipment. — In  the  cases  of  such  artificial  gases  as  pro- 
ducer gas,  water  gas,  etc.,  the  cost  of  the  necessary  plant  equipment 
for  their  manufacture,  besides  interest  and  depreciation,  must  also 
be  figured  in  with  the  cost  of  the  gas.  It  is  evident  that  the  heat- 
treatment  plant  must  be  of  considerable  size  to  warrant  a  large 
initial  plant  investment  for  the  manufacture  of  such  gas.  Similarly, 
if  oil  is  to  be  used,  there  must  also  be  an  allowance  for  storage  tanks 
for  protection  against  poor  deliveries  and  unusual  service  demands. 
The  cost  of  piping  for  fuel  distribution  is  lower  in  the  case  of  oil 
than  with  gas,  as  smaller  pipes  are  used. 

Fuel  Supply.- — The  selection  of  a  fuel  involves  not  only  its  heat- 
unit  value,  but  a  consideration  of  local  conditions  of  constancy  of 
supply  and  of  sufficient  quantity  without  undue  fluctuation  in  price. 
The  source  of  fuel  supply  must  be  absolutely  dependable,  both  in 
regularity  of  deliveries  and  uniformity  of  fuel.  This  item  is  largely 


182  STEEL  AND   ITS  HEAT  TREATMENT 

connected  with  the  use  of  oil  and  is  a  phase  of  the  fuel  question  well 
worth  studying.  Thus,  in  considering  the  availability  of  any  fuel, 
the  locality,  natural  resources  and  freight  should  be  taken  into 
account. 

Uniformity  of  Fuel. — High  quality  of  product  requires  uniformity 
of  heating,  and  which  in  turn  requires  uniformity  of  fuel.  Such  a 
product  may  be  obtained  only  by  having  an  absolute  control  of  the 
volume,  temperature  and  composition  of  the  gases  to  and  from  all 
points  of  the  chamber.  Any  changes  taking  place  in  the  composition 
of  the  fuel  not  only  makes  the  work  of  the  furnace  man  more  difficult 
and  exacting,  but  also  must  inevitably  result  in  intermittent  heating 
and  unsteadiness  of  operation.  That  is,  variable  supply  or  non- 
uniform  fuel  is  not  conducive  to  good  product.  In  this  connection 
it  is  advisable  to  note  that  the  presence  of  sulphur — as  in  producer 
gas  or  coal — is  more  or  less  injurious  to  the  metal  when  at  high 
temperatures,  as  it  has  been  shown  that  hot  steel  is  capable  of  being 
sulphurized  as  well  as  carburized. 

Fuel  Oil. — There  is  probably  no  one  fuel  that  has  been  more 
abused  than  fuel  oil.  Its  great  concentrated  heat  value  and  flexibility 
of  application  and  control  have  been,  commercially,  its  greatest  draw- 
back, for  the  reason  that  these  advantages  have  permitted  the  applica- 
tion of  the  fuel  in  a  haphazard  manner  by  people  who  seem  to  be 
satisfied  so  long  as  it  was  burned  and  made  heat  in  some  form  or 
other.  Even  many  of  the  manufacturers  of  oil-burning  equipment 
are  not  entirely  free  from  this  criticism.  Much  of  the  competition 
which  has  been  held  against  fuel  oil,  and  comparative  statements  of 
operating  costs  that  have  been  made  up,  are  not  based  on  an  efficient 
use  of  the  fuel  at  all. 

Air  Control  with  Fuel  Oil. — The  majority  of  heating  equipment 
installed  with  oil  are  lacking  in  some  of  the  very  elementary  essen- 
tials necessary  for  good  combustion,  and  in  this  respect  are  not 
anywhere  near  as  efficient  as  an  ordinary  kitchen  stove.  There  seems 
to  be  an  absolute  disregard  of  the  fact  that  it  is  just  as  necessary 
to  control  all  of  the  air  entering  into  a  furnace  with  oil  fuel  as  it  is 
with  gas  or  coal.  Most  city  gas  equipment  has  this  provision,  as 
have  boilers  or  ordinary  stoves,  but  it  seems  to  be  entirely  lacking 
with  most  oil-burning  equipment,  although  the  reasons  for  it  are  just 
as  important  in  one  case  as  in  another.  The  common  practice  is 
to  inject  oil  in  a  hole  through  the  side  of  a  furnace;  and  many  people 
think  that  because  there  is  a  valve  on  the  oil  and  air  lines  that  they 
consequently  control  the  air.  This,  however,  is  not  true,  for  the 


HEAT  GENERATION  183 

reason  that  in  many  furnaces  by  far  the  greater  proportion  of  air 
required  for  the  combustion  of  the  fuel  is  "  induced  "  by  the  force  of 
the  blast  and  does  not  pass  through  the  burner  itself.  It  has  been 
by  reason  of  such  conditions  that  oil  has  been  abandoned  and  given 
a  bad  name  in  many  places  where  the  conditions  would  be  reversed 
if  it  were  properly  handled. 

The  Human  Element. — Thus  the  average  man,  when  he  sees  a 
kerosene  lamp  smoke  and  blacken  the  chimney,  or  a  gas  mantle  puff 
and  impair  the  light,  will  ordinarily  recognize  that  something  is 
wrong,  and  immediately  make  the  adjustments  necessary  to  over- 
come the  difficulty.  Both  of  these  are  the  effects  of  a  common  cause 
— the  improper  mixture  of  the  fuel  and  air  necessary  for  proper 
combustion;  and  in  making  such  adjustments,  whether  he  knows 
it  or  not,  he  is  merely  establishing  the  proper  relationship  between 
the  fuel  and  air  necessary  for  good  combustion. 

Yet  we  will  find  this  selfsame  man,  day  after  day  and  year  after 
year,  operate  or  permit  others  to  operate,  at  great  expense,  furnaces 
with  oil  or  gas  which  smoke  and  puff  and  pollute  the  atmosphere 
with  hot  and  obnoxious  gases,  but  never  think  of  making  the  ad- 
justments necessary,  which  are  the  same  as  those  required  in  case  of 
the  lamp.  Such  a  man  is  either  not  "  on  the  job  "  or  his  furnace  is 
lacking  in  the  essentials  for  good  combustion  common  to  every  house- 
hold lamp  or  stove,  whether  it  burn  oil  or  gas  or  coal.  Yet  the 
majority  of  men  employed  in  heat-treating  work,  as  well  as  a  large 
percentage  of  furnaces,  particularly  those  fired  with  oil  or  coal,  are 
open  to  this  criticism,  which  is  evidence  of  the  necessity  for  improve- 
ment in  the  personal  element,  at  least  to  the  extent  of  either  making 
the  adjustments  if  provision  for  such  is  on  the  furnaces,  or  at  least  to 
insist  that  the  furnaces  be  designed  on  the  A.B.C.  principles  of  heat 
generation.  And  when  such  adjustments  are  made  with  proper 
furnaces,  the  operator  benefits  himself  by  decreasing  the  heat  and 
gases  affecting  his  health  and  comfort,  and  benefits  his  employer  in 
turning  out  better  product  and  saving  fuel  and  power  and  conserving 
the  life  of  his  furnace.  Such  conditions,  which  actually  exist  in  the 
majority  of  shops  in  the  country,  are  a  sad  commentary  on  the  work 
of  efficiency,  safety  first  and  industrial  betterment  ideas  so  prevalent 
at  this  time,  and  sustain  the  point  that  we  often  look  at  and  do  not  see 
opportunities  for  improvement  that  can  be  made  in  a  simple  way. 
The  average  furnace  operator  appears  to  act  on  the  principle  that  he 
is  not  making  a  good  showing  unless  he  has  plenty  of  smoke  and  flame 
belching  out  of  every  crevice  of  the  furnace — probably  for  the  same 


184  STEEL  AND  ITS  HEAT  TREATMENT 

reason,  or  lack  of  reason,  responsible  for  the  blacksmith  striking  two 
blows  on  the  anvil  to  one  on  the  horseshoe. 

It  would  appear  only  reasonable  to  assume  that  the  existence  of 
such  conditions  in  the  shop  does  not  permit  the  owner  of  such  shop 
to  make  the  statement  that  one  of  his  most  important  manufacturing 
operations — i.e.,  the  heat  treatment  of  good  steel — is  conducted  under 
the  best  possible  methods  with  the  best  possible  furnace  equipment 
by  the  highest  grade  men,  or  that  it  is  even  on  a  par  with  the  average 
machine-shop  practice.  And  all  this  notwithstanding  the  evidence 
that  may  be  offered  in  the  way  of  "  Temperature  Records  "  or 
"  Fuel-burning  Equipment  "  in  an  attempt  to  sustain  the  point. 

It  is  invariably  cheaper  to  do  it  right.  The  extra  expense  in  solved 
in  furnaces  of  heavy  construction  with  proper  provision  for  applying 
heat  to  the  chamber  and  for  removing  it  from  the  chamber,  is  insig- 
nificant when  compared  with  the  saving  effected. 

The  Value  of  the  Furnace  Operator. — If,  as  generally  conceded, 
men  are  paid  in  proportion  to  their  skill  and  the  part  that  such  skill 
plays  in  the  make-up  of  a  finished  product,  then  a  good  annealer  is 
worth  more  than  a  roller  in  a  rolling  mill,  and  a  good  man  in  charge 
of  heat-treatment  work  is  worth  more  than  an  automatic  machine 
operator  in  an  automobile  or  machine  shop.  In  one  case  the  man 
operates  a  machine  and  it  is  a  machine  that  more  or  less  determines 
the  result,  and  at  any  rate  they  are  a  mechanical  check  on  the  opera- 
tion. In  the  other  case  it  is  purely  a  question  of  skill,  experience, 
and  judgment,  with  no  mechanical  check  upon  the  major  part  of  the 
operation.  The  furnace  and  all  auxiliary  appliances  are  but  tools, 
and  while  it  is  necessary  that  they  should  be  of  the  best,  they  are 
nevertheless  but  tools  in  effect  and  bear  about  the  same  relation  to 
the  result  that  a  good  tool  in  the  hands  of  a  good  operator  bears  to 
any  other  result.  Furthermore,  it  is  the  judgment  of  the  furnace 
operator  that  determines  if  the  work  of  all  that  have  preceded  him 
shall  be  spoiled  or  improved  upon,  and  if  the  time,  labor,  and  money 
spent  to  produce  the  results  sought  for  are  capitalized  or  wasted. 

Purchasing  Brains. — It  has  been  stated  that  the  average  manu- 
facturer will  unhesitatingly  invest  money  in  anything  involved  in 
his  processes  of  manufacture  outside  of  the  human  element,  which  is 
virtually  paying  a  premium  on  everything  but  brains.  It  is  common 
practice  to  install  an  expensive  machine,  costing  thousands  of  dollars, 
and  employ  a  cheap,  inefficient  man  to  run  it,  notwithstanding  that 
it  is  the  man  who  controls  the  output  and  cost  of  operating  that 
machine.  When  the  relationship  of  the  human  element  to  the  result 


HEAT  GENERATION  185 

is  of  less  importance,  as,  for  instance,  with  a  machine  press — as  it 
surely  is  when  compared  with  a  furnace — then  there  will  be  mani- 
fested the  false  economy  effected  by  the  employment  of  unskilled, 
inefficient  operators.  In  the  final  analysis  the  machine  or  the  fur- 
nace is  nothing  more  than  a  tool  in  the  hands  of  the  operator,  and 
the  value  of  the  human  element  depends  upon  the  amount  of  skill 
that  must  be  exercised  in  the  use  of  a  tool,  whether  it  be  a  hammer, 
a  chisel,  a  press  or  a  furnace.  There  are  many  cases  when  it  is  per- 
missible to  employ  unskilled  labor  in  connection  with  a  machine, 
where  the  operation  is  more  or  less  automatic  and  the  operator  is 
required  to  do  nothing  more  than  start  or  stop  the  movement  and 
feed  material.  Such  a  practice,  however,  is  foolhardy  with  a  furnace, 
because  of  the  paramount  importance  of  the  human  element  in  the 
operation.  It  is  a  waste  of  money  to  install  efficient  types  of  fur- 
naces, which  are  necessarily  expensive,  without  intelligent  super- 
vision over  the  operation  in  the  form  of  at  least  one  efficient  man  who 
can  either  operate  it  himself  or  direct  its  operation  by  others.  The 
practice  of  employing  at  least  one  skilled  man  for  such  purpose  is 
gaining  headway  and  will  undoubtedly  continue  to  do  so,  as  his  labor 
is  usually  more  than  paid  for  by  the  savings  effected  in  the  cost  of 
operation,  to  say  nothing  of  the  betterment  of  the  product.  A  good 
furnace  coupled  with  a  poor  operator  does  not  make  the  proper 
combination,  and  when  both  are  of  inferior  caliber,  as  they  so  often 
are,  then  it  is  unreasonable  to  suppose  that  the  all-important  heating 
operations  are  conducted  as  they  should  be,  even  though  there  may 
be  some,  but  nevertheless  weak,  evidence  in  the  form  of  pyrometer 
records  to  the  contrary. 

Effect  of  Operation. — The  operation  of  the  furnace  in  the  shop 
should  be  regarded  in  the  same  light  as  the  stove  in  the  kitchen. 
The  furnace  operators  should  be  taught  that  furnaces  operate  on  the 
same  principles  as  an  ordinary  house-heating  coal  stove;  both  must 
be  given  the  proper  attention  in  the  matter  of  regulating  the  dampers 
for  the  air  supply  and  flue  gases  in  order  to  accomplish  the  same 
results.  The  importance  of  regulating  the  amount  of  air  which  is 
used  for  the  combustion  of  the  fuel  cannot  be  too  strongly  emphasized. 
In  this  respect  the  ordinary  kitchen  stove  is  in  many  ways  superior 
to  many  so-called  heart-treatment  furnaces.  Just  study  the  cook- 
stove  in  your  own  home  and  see  how  many  different  ways  there  are 
for  adjusting  the  air  supply  and  draft.  To  illustrate:  under  the  fire 
there  are  usually  one  or  possibly  two  dampers ;  directly  over  the  fire 
there  is  another  damper  for  checking  the  fire;  another  damper  will 


186  STEEL  AND   ITS  HEAT  TREATMENT 

permit  of  the  carrying  of  the  heat  currents  around  the  oven  (conser- 
vation) ;  and  still  another  damper  or  two  will  shut  off  the  draft  up 
the  chimney.  As  a  general  question,  how  many  heat-treatment 
furnaces  have  an  equal  number  of  devices  for  controlling  the  combus- 
tion or  heating?  Apart  from  the  question  of  recuperation  and  its 
direct  saving  in  fuel,  there  is  involved  the  method  of  operation, 
and  question  of  furnace  design  which  in  itself  is  modified  by  the 
method  of  operation.  While  in  some  cases  the  application  of  the 
fuel  is  poor,  in  others  the  construction  of  the  furnace  may  be  at 
fault  or  not  adapted  to  the  work  it  has  to  do. 

Furnace  Design. — Poorly  designed  furnaces  are  the  cause  of  much 
of  the  difficulty  and  time  spent  in  trying  to  account  for  poor  results 
which  could  be  better  spent  in  insuring  better  conditions.  Many 
furnaces  to-day  are  operated  with  elaborate  pyrometer  systems,  but 
with  a  complete  disregard  for  the  conditions  that  make  good  heat- 
ing possible.  There  is  no  control  of  the  air  for  combustion  entering 
the  furnace,  no  control  of  the  waste  gases  leaving  the  furnace,  and  no 
knowledge  of  their  composition  or  action  while  in  the  furnace.  Under 
such  conditions  good  combustion,  with  soft  heats  and  uniformly 
heated  stock,  is  out  of  the  question.  The  object  is  not  merely  to 
burn  fuel,  or  to  make  heat,  but  to  apply  the  heat  economically  and 
effectively. 

Oil  Burners. — There  is  altogether  too  much  importance  attached 
to  oil  burners,  both  by  manufacturers  of  such  appliances  as  well  as 
by  users.  Too  many  people  have  the  idea  that  all  that  is  necessary 
to  do  with  oil  is  to  buy  an  "  efficient  "  burner  and  build  a  furnace 
around  it.  But  there  is  really  no  such  thing  as  an  oil  burner  in  the 
sense  usually  taken  for  a  gas  burner.  The  very  term  is  a  misnomer, 
as  the  oil  burner  is  nothing  more  than  a  valve  and  its  efficiency  is 
mechanical  and  not  thermal.  In  fact,  the  majority  of  oil  burners 
are  not,  properly  speaking,  mixing  valves,  as  most  gas  burners  are, 
for  the  reason  that  the  most  successful  burners  are  nothing  more 
than  valves  which  introduce  fuel  and  air  into  the  furnace  in  pro- 
portions fixed  by  the  operator,  the  actual  mixing  taking  place  in  the 
furnace  and  not  in  the  burner  itself.  It  does  not  matter  as  much 
how  the  fuel  is  delivered  at  the  furnace  as  what  is  done  with  it 
after  it  is  delivered  in  the  furnace. 

Burner  vs.  Furnace. — This  influence  which  the  design  of  the  fur- 
nace— that  is,  the  method  of  using  the  fuel  in  the  furnace — has  upon 
the  economy  of  heating,  regardless  of  the  burner,  is  well  brought 
out  by  the  following  chart,  to  accompany  Figs.  94,  95  and  96. 


PROGRESS  IN  ROTARY  FURNACE  CONSTRUCTION.          187 

Three  different  types  doing  the  same  work. 


FIG.  94. — Externally  Fired  Cast-iron 
Tumbling-barrel  Furnace  (1898). 


FIG.  95. — Externally  Fired  Cast-iron  Helical-cylinder  Furnace  (1906). 


Pyrometer  Connection 

Steam    |||     /  ,__-,      Discharge  Hood 

or  Air- 


Charging  Drum 


FIG.  96. — Internally  Fired  Tile-lined  Helical-cylinder  Furnace  (1909). 


188 


STEEL  AND   ITS  HEAT  TREATMENT 


The  figures  are  taken  from  actual  operation  and  weight  of  material 
and  fuel. 


Design  of  Furnace. 

Externally  Fired 
Cast-iron  Tumbling- 
barrel  Furnace, 
1898. 

Externally  Fired 
Cast-iron  Helical- 
cylinder  Furnace, 
1906. 

Internally  Fired 
Tile-lined  Helical- 
cylinder  Furnace, 
1909. 

Metal  heated 
per  hour 

287  pounds 

576  pounds 

1450  pounds 

Increase  of  metal 
heated  per  hour 

100  per  cent. 

405  per  cent. 

Fuel  oil  burned 
per  hour 

6.41  U.  S.  gals. 

3.64  U.  S.  gals. 

3.48  U.  S.  gals. 

Decrease  of  fuel 
oil  burned  per  hour 

43.3  per  cent. 

45.7  per  cent. 

Metal  heated  per 
gallon  of  fuel  oil 

45  pounds 

158  pounds 

416  pounds 

In  each  case  the  pieces  heated  were  of  the  same  size  and 
material  and  the  lightest  in  individual  weight  of  their  kind.  The 
figures  represent  three  different  types  of  rotary  heating  furnaces 
doing  the  same  work.  The  advantage  in  favor  of  the  internally 
fired  helical  furnace — Fig.  96 — was  still  more  marked  when  heating 
pieces  of  greater  individual  weight.  The  material  was  also  of  better 
quality,  being  freer  from  oxidation.  The  cost  of  repairs  is  also  very 
much  less  for  the  internally  fired  helical  furnace  than  either  of  the 
others. 

Now  these  three  furnaces  were  operated  with  the  same  burner, 
the  same  fuel  oil,  the  same  steam  pressure  for  atomizing,  the  same 
air  for  combustion,  the  same  material,  the  same  temperature,  at  the 
same  time,  and  by  the  same  men.  This  then  illustrates  the  point 
that  it  is  the  furnace  and  not  the  burner  alone  which  produces  the 
desired  results. 

"  Quality  of  Heat." — Much  has  been  said  about  "  the  quality 
of  heat";  but  as  heat  can  differ  only  in  the  degree  of  temperature, 
such  a  statement  must  therefore  refer  to  the  atmosphere  in  the  fur- 
nace. One  of  the  most  recent  books  on  the  subject  of  heat  treatment 
of  steel  has  the  following  statements : 

"  Until  recently,  the  only  known  way  of  producing  heat  of  the 
required  intensity  was  by  combustion — the  burning  of  some  fuel. 
The  attendant  disadvantages  of  this  are  well  known.  The  crude 


HEAT  GENERATION  189 

open  coal  forge  is  capable  of  heating  the  steel,  but  leaves  much  to  be 
desired  as  regards  the  quality  of  the  heat,  its  uniformity,  and  the 
temperature  control.  In  order  to  produce  heat  at  all,  the  carbon 
in  the  coal  must  be  combined  with  the  oxygen  of  the  air,  and  a 
strongly  oxidizing  flame  is  unavoidable.  The  steel  exposed  to  this 
action,  or  to  the  inevitable  results  of  it,  suffers  accordingly.  The 
coke-burning  furnace  offered  some  improvements,  but  only  in  detail. 
Now  there  are  highly  perfected  furnaces  for  burning  oil  and  gas, 
and  some  of  these  offer  still  further  advances,  but  the  principle  at 
the  basis  of  all  of  these  is  the  same — there  must  be  a  '  burning  ' 
process  to  produce  the  heat;  oxidation  must  be  present  with  all  fuel- 
combustion  furnaces. 

"  Through  what  means,  then,  may  we  obtain  the  proper  quality 
of  heat,  uniformly  applied,  and  of  the  right  degree?  The  electric 
furnace  for  the  heating  of  steel  brings  the  answer.  It  overcomes 
most  of  the  objections  to  the  '  combustion  process '  by  introducing 
a  new  principle." 

Further  on  the  statements  are  made:  "The  atmosphere  in 
the  heating  chamber  of  the  electric  furnace  is  inherently  l  reducing  ' 
in  its  nature,  due  to  the  fact  that  the  hot  carbon  plates  absorb  all 
of  the  atmospheric  oxygen.  By  raising  the  door  slightly,  and  open- 
ing the  draft-hole  at  the  rear,  a  slight  current  of  air  may  be  admitted 
which  will  counteract  this  tendency.  Leaving  the  door  open  slightly 
more  would  allow  an  excess  of  air  to  enter,  so  that  an  oxidizing 
atmosphere  could  be  produced.  Between  the  extreme  points  fine 
shades  of  atmospheric  conditions  can  be  obtained.  Thus  the  qual- 
ity of  the  heat  can  be  absolutely  and  easily  regulated." 

Furnace  Atmospheres  from  Combustion. — Commenting  on  the 
above,  the  question  of  atmosphere  in  the  heating  chamber  is  one  of 
operation  of  the  furnace,  assuming  the  proper  furnace  design,  and 
simply  comes  down  to  the  relation  between  the  fuel  and  the 
air  supply.  In  the  design  and  operation  of  the  best  types  of 
heating  furnaces  it  is  the  aim  to  produce  an  atmosphere,  under 
pressure,  which  virtually  contains  no  oxygen  and  only  a  very 
slight  amount  of  reducing  elements.  Take  for  example  the 
under-fired  type  of  furnace,  properly  designed  and  operated. 
The  actual  combustion  of  the  fuel  takes  place  in  the  separate 
chamber  under  the  hearth  where  the  amount  of  air  taken  into 
the  furnace  is  just  sufficient  to  produce  perfect  combustion  of 
the  fuel.  By  the  time  the  hot  gases  actually  reach  the  heating 
chamber  they  are  thoroughly  mixed  and,  on  account  of  the  design 


190  STEEL  AND   ITS  HEAT  TREATMENT 

of  the  furnace,  a  pressure  is  built  up.  This  pressure  stops  any  inflow 
of  free  oxygen  through  the  doors,  and  otherwise  surrounds  the  steel 
with  the  neutral  atmosphere  of  hot  gases.  That  these  hot  gases 
may  actually  contain  no  oxygen  and  very  little  reducing  vapors  is 
well  shown  by  the  following  analysis : 

Per  Cent. 

Carbon  dioxide 12.5 

Oxygen 0 

Oil  vapors 0 

Carbon  monoxide 2.1 

Nitrogen 85.4 

From  a  study  of.  this  analysis,  which  was  taken  under  ordinary 
operating  conditions  of  a  well  designed  furnace  and  without  the 
operator  knowing  that  anything  unusual  was  expected  of  him,  it  cer- 
tainly cannot  be  said  that  the  "  combustion  process  "  produces, 
under  proper  conditions,  anything  but  a  proper  neutral  atmos- 
phere. 

Furnace  Atmospheres  with  Electricity. — In  contrast  with  this, 
consider  the  effect  taking  place  in  the  electric  furnace  previously 
referred  to.  In  this  case  the  heat  is  virtually  supplied  outside  of, 
and  through,  the  walls  of  the  chamber.  Free  access  of  unaltered 
atmospheric  air  to  the  inside  of  the  furnace  exists,  and  coming  in 
contact  with  the  heated  steel  results  in  a  very  rapid  oxidation  of  the 
metal.  An  attempt  is  sometimes  made  to  remedy  this  condition 
by  introducing  charcoal  into  the  chamber  with  which  the  oxygen 
is  supposed  to  combine  to  form  an  inert  atmosphere  of  carbon 
dioxide,  or  even  by  exposing  the  carbon  electrodes  of  the  electric 
element  and  allowing  the  oxygen  present  in  the  chamber  to  attack 
them  and  thus  consume  the  free  oxygen.  It  must  be  realized  that 
if  this  elimination  of  the  free  oxygen  thus  occurs  it  must  take  place  in 
the  furnace  itself  and  in  the  chamber  where  the  stock  is  being  heated 
and  the  steel  is  therefore  more  or  less  exposed  to  oxidation. 

The  argument  previously  advanced  that  the  atmosphere  in  the 
electric  furnace  may  be  controlled  by  slightly  raising  the  door  and 
opening  a  vent  in  the  back  of  the  furnace  is  entirely  contrary  to  heat- 
ing principles.  The  cost  of  the  heating  itself  advances  when  this 
takes  place  because  cold  air  is  entering  through  the  front  and  the 
heat  is  allowed  to  flow  out  through  the  vent.  Such  a  proposition 
is  analogous  to  the  effort  of  trying  to  heat  a  house  in  the  winter 
with  the  doors  open. 


HEAT  GENERATION  191 

Electricity  for  Heating. — The  advantages  of  electricity  as  a 
source  of  heat  compared  with  oil  or  vice  versa  are  determined 
only  by  the  nature  of  the  operation  regardless  of  fuel  cost.  Each 
form  of  energy  has  its  own  field  of  use.  When  it  is  considered  that 
fuel  oil  under  proper  application  is  continually  being  used  within 
temperature  limits  of  10°  F.,  it  must  be  evident  that  electricity  must 
offer  some  powerful  indirect  advantages  for  a  great  deal  of  heat 
treatment  work  in  order  to  be  given  a  chance.  On  the  other  hand, 
there  are,  however,  many  cases  like  the  open-hearth,  reverberatory 
and  other  forms  of  heating  where  the  limit  has  been  about  reached 
with  fuel.  It  is  in  such  operations  where  electricity,  by  reason  of  a 
better  method  of  applying  heat  to  the  stock,  can  overcome  the 
disadvantage  of  higher  fuel  cost  with  less  actual  energy  for  the  opera- 
tion. Thus  we  might  also  designate  certain  forms  of  chain  welding 
as  good  examples  where  electricity  might  be  advantageous. 

Fuel  vs.  Product. — Reduced  to  lowest  terms,  heat  generation  is 
an  economic  problem.  Commercial  heat  treatment  requires  the 
production  of  the  best  attainable  results  at  the  lowest  cost.  We 
may  sum  up  the  fuel  question  by  stating  that  in  connection  with 
appropriate  furnace  design  and  furnace  operation,  that  fuel  should  be 
used  which  will  cost  the  least  viewed  from  the  standpoint  of  finished 
product.  This  cost  not  only  applies  to  increased  output,  but  also  to 
the  quality  of  the  product.  The  former  results  in  lower  manufactur- 
ing costs;  the  latter  in  greater  efficiency  and  higher  selling  prices. 


CHAPTER  IX 
HEAT   APPLICATION 

Furnace  Equipment  in  General.— The  usual  consideration  of 
furnace  equipment  for  heat-treatment  operations  is  based  on  require- 
ments of  "  accurate  temperature/7  "  heat  control,"  "  temperature 
variation,"  etc.,  with  the  result  that  the  trade  literature — which  is 
the  catechism  of  many,  if  not  of  the  majority  interested  in  the  work — 
has  developed  a  standard  of  heating  which  is  altogether  too  low  and 
must  be  superseded  by  one  based  on  a  broader  view  of  the  problem 
in  order  to  effect  greater  progress. 

The  existence  of  such  a  standard  is  probably  due  to  the  reasoning 
— "  that  a  uniformly  heated  piece  naturally  involves  a  uniform 
temperature  within  the  furnace,  and  the  less  the  variation  in  tem- 
perature the  better  the  results  will  be;  therefore,  in  order  to  produce 
a  uniformly  heated  product  it  is  necessary  to  employ  a  furnace  that 
will  produce  a  uniform  heat  with  minimum  variation  of  tempera- 
ture." The  natural  result  of  such  reasoning  has  been  a  development 
of  pyrometers  to  indicate  the  variations  in  temperature,  and  the 
development  of  many  different  designs  of  furnaces  which  have  been 
vigorously  and  extensively  exploited  on  the  strength  of  claims  for 
"  accurate  temperature  control,"  "  minimum  temperature  varia- 
tion," "  neutral  or  reducing  flame  as  desired,"  "  no  oxidation,"  etc. 

That  phase  of  the  work  covering  pyrometry  has  been  of  ines- 
timable value  and  is  more  highly  developed  than  nearly  any  other 
branch  of  heat-treatment  work.  It  is,  however,  unfortunately  too 
often  considered  as  an  end  when  in  reality  it  is  but  a  means  to 
an  end — a  means  which  if  properly  employed  will  indicate  the  simple 
principles  back  of  "  heat  application,"  as  in  comparison  with  "  heat 
generation  "  or  "  heat  utilization." 

The  standard  of  heating  requirements,  with  a  means  in  the  form 
of  pyrometry  to  check  them,  has  developed  much  discussion  of  the 
relative  merits  of  the  different  designs  of  furnaces  offered  to  meet 
them.  Much  of  this  has  been  directed  towards  the  claims  made  for 
the  furnaces  supported  with  evidence  in  the  form  of  pyrometer  charts, 

192 


HEAT  APPLICATION  193 

heat  logs,  and  other  data  incident  to  the  indication  of  heat,  tending 
to  show  the  ability  of  the  various  designs  to  meet  the  heating  require- 
ments above  outlined.  This  is  all  right  as  far  as  it  goes,  but  it  does 
not  go  far  enough.  It  may  be  considered  as  "  evidence  "  of  a  heat 
condition  in  some  part  of  a  furnace  chamber,  but  not  necessarily 
as  "  proof  "  of  a  uniformly  heated  product  within  that  furnace 
chamber.  If  it  were  otherwise,  then  we  would  have  not  to  deal  so 
often  with  variations  in  the  finished  product  without  any  apparent 
variation  in  the  indicated  temperature.  Heating  a  furnace  chamber 
uniformly,  and  uniformly  heating  a  product  within  that  furnace 
chamber,  are  two  distinct  operations.  The  former  must  accompany 
the  latter,  but  the  mere  indication  of  the  former  does  not  by  any 
means  prove  the  existence  of  the  latter.  It  does  not  follow  that  the 
temperature  variation  indicated  in  any  two  points  in  a  chamber 
when  empty  will  be  the  same  as  that  indicated  when  the  chamber  is 
partially  filled,  or  more  particularly,  when  it  is  filled  to  full  normal 
capacity.  The  real  test  of  a  furnace  for  a  given  operation  from  the 
standpoint  of  uniformly  heated  product,  is  not  the  temperature 
variation  when  the  chamber  is  empty  or  partially  filled,  but  the 
temperature  variation  around  the  mass  to  be  heated  when  the  chamber 
is  loaded  to  full  capacity. 

This  is  a  simple  fundamental  rule  which,  when  considered  with 
reference  to  a  given  operation,  illustrates  why  it  is  possible  to  secure 
better  results  in  finished  product  with  one  type  of  furnace  over 
another  without  any  apparent  difference  in  the  indicated  variation 
in  temperature  at  any  given  point. 

To  illustrate:  Let  us  consider  two  gas-fired  bake-ovens  of  the 
same  size,  each  designed  to  accommodate  six  cakes  of  a  given 
size,  the  essential  difference  between  the  two  being  that  one 
is  heated  by  gas  jets  at  the  bottom,  as  in  Fig.  97  (a),  and  the 
other  by  gas  jets  at  the  top,  as  in  (6),  the  problem  being  the 
uniform  heating  or  baking  of  the  six  cakes  to  be  placed  on  the 
tray  x. 

If  a  thermo-couple  should  be  introduced  at  any  two  points  on  the 
tray  x  in  the  empty  ovens,  it  will  be  found  that  the  heat  will  be 
fairly  uniform  with  either  design,  even  though  with  one  design  the 
actual  oven  temperature  may  be  different  from  the  other.  It  is 
reasonable  to  suppose  that  one  small  cake,  as  in  (c),  would  compare 
favorably  with  (d) ,  as  there  is  ample  room  for  circulation  of  the  heat 
from  one  side  to  the  other.  We  have  in  one  case  (a  or  b)  an  indi- 
cation of  a  uniform  chamber  temperature,  and  in  the  other  (c-d) 


194 


STEEL  AND   ITS  HEAT  TREATMENT 


an  indication  of  a  uniform  chamber  temperature  and  of  a  uniformly 
heated  product. 

Now  suppose  that  each  oven  is  filled  to  normal  capacity,  as  in 
(e)  or  (/).  If  the  space  between  the  pieces  is  small,  it  is  very  likely 
that,  in  a  given  time,  a  given  temperature  will  produce  a  color  on  the 
side  of  the  cake  nearest  the  fire  that  will  be  different  from  the  color 


(a) 


nnnnnn 


FIG.  97. 

on  the  opposite  side.  It  is  also  likely  that  the  variation  between 
any  two  points  on  the  same  side  of  the  tray  will  be  very  slight,  but 
that  the  variation  between  the  under  and  upper  sides  of  the  tray 
will  be  considerable.  This  indicates  the  possibility  of  turning  out 
a  product  not  uniformly  heated  from  a  chamber  that  may  be  uni- 
formly heated  when  empty,  or  but  partially  filled,  and  at  the  same 
time  constantly  indicating  a  uniform  temperature  on  any  lateral 
plane. 


HEAT  APPLICATION  195 

This  illustrates  how  weak  and  irrelevant  such  claims  as  previously 
noted  are  to  the  real  question — the  uniformity  of  the  heated  product. 
The  very  manner  of  placing  the  stock  in  the  chamber  may  affect  the 
final  result.  Uniform  temperature  in  the  chamber  is  a  part  but  not 
all  of  the  process.  It  is  the  peculiar  conditions  or  requirements  of 
each  case  that  determine  the  type  of  furnace  to  use  and  the  manner 
of  heating  the  steel  in  it.  There  is  no  such  thing  as  an  ideal  furnace 
for  heat  treatment  any  more  than  there  is  an  ideal  engine  or  fuel  for 
power,  for  in  one  case  as  in  the  other,  the  point  is  determined  by  the 
working  conditions. 

Heat  Application. — Heating  a  piece  of  steel,  boiling  a  cup  of  water 
or  baking  a  potato  are  alike  heat-treating  operations,  and  in  so  far 
as  each  leads  to  an  absorption  of  heat,  they  are  comparable.  A  cup 
of  water  will  boil  much  sooner  if  the  heat  is  applied  from  the  bottom 
rather  than  from  the  top  downwards.  In  the  ordinary  gas  stove 
the  oven  is  heated  from  jets  below,  while  these  same  jets  deflected 
downwards  heat  the  broiler  from  above;  a  potato  in  the  oven  is 
heated  evenly  through  to  the  center  without  burning  the  outside, 
but  a  potato  placed  on  the  broiler  is  burned  to  a  crisp  on  the  top 
while  the  center  remains  hard  and  uncooked. 

Many  carburizing  boxes  give  evidence  of  having  been  "  broiled  " 
rather  than  "  baked."  Boys  still  roast  potatoes  in  open  bonfires; 
men  still  heat  steel  in  open  smith-fires;  and  the  results  are  about  the 
same.  An  inefficient  plant  means  that  the  product  seldom  reaches 
and  never  maintains  the  standard  to  which  it  is  entitled,  and  also 
that  the  cost  of  up-keep  and  labor  far  outweigh  any  difference  in  first 
cost  of  plant. 

To  make  results  harmonize  with  the  requirements  in  each  case 
requires  furnaces  of  the  best  possible  construction,  fuel  that  would 
cost  the  least  viewed  from  the  standpoint  of  finished  product, 
and  it  further  requires  that  these  furnaces  be  so  arranged  and  such 
methods  devised  that  the  material  may  be  heated  and  handled  with 
the  least  labor  and  loss  of  time. 

Only  when  this  harmonious  combination  of  suitable  furnaces, 
right  fuel,  proper  furnace  layout  and  efficient  material  handling 
conditions  has  been  secured  can  there  be  accomplished  the  best 
heating  of  a  product  at  the  least  total  cost. 

The  "  One  "  Furnace. — One  often  reads  of  the  claims  of  furnace 
manufacturers  that  this  or  that  furnace  is  the  "  only  one  "  for  heat 
treatment.  This  is  all  wrong,  as  there  is  no  one  type  of  furnace  for 
heating  any  more  than  there  is  one  type  of  building  for  machine 


196  STEEL  AND  ITS  HEAT  TREATMENT 

operations.  There  are  certain  principles  of  construction,  heat 
generation,  application  or  utilization  that,  when  properly  combined, 
make  up  the  right  furnace;  but  it  is  always  the  local  shop  conditions 
that  determine  how  these  combinations  should  be  effected.  There 
is  as  much  more  experience  and  skill  required  for  the  determination 
of  a  furnace  design  than  in  building  it,  as  there  is  between  an  architect 
and  builder  or  carpenter  in  the  design  and  erection  of  a  building. 

Furnace  Guarantees. — One  would  not  think  of  asking  a  stove 
manufacturer  to  guarantee  good  cooking  with  his  stove,  and  it  is  as 
unreasonable  to  expect,  as  it  is  foolish  to  offer,  a  furnace  guaranteed 
for  good  heat  treatment  without  the  proper  handling.  The  right 
furnace,  like  the  right  stove,  only  makes  it  possible;  and  it  is  the  man, 
like  the  cook,  that  determines  the  final  result.  What  is  most  needed 
at  this  time  is  a  better  appreciation  of  heat  application  to  useful 
work,  as  removed  from  heat  generation,  combustion  or  utilization. 
There  should  be  more  study  given  to  the  absorption  of  heat  by  the 
product  and  the  manner  of  placing  and  handling  to  secure  the  best 
heating  results. 

Uniform  Heating. — The  problem  of  applying  heat  to  industrial 
work  is  but  a  cooking  operation,  like  the  baking  of  bread;  the  bread 
must  be  heated  uniformly  throughout.  Similarly  the  charge  in  a 
heat-treatment  furnace  may  be  best  heated  when  it  has  opportunity 
to  absorb  heat  uniformly  from  all  sides.  And  as  far  as  the  heat 
absorption  is  concerned  it  does  not  matter  whether  this  heat  is 
supplied  as  radiating  heat  from  the  lining  of  the  furnace  or  through 
the  direct  application  of  hot  gases.  Except  in  the  case  when  elec- 
tricity is  used  as  the  source  of  heat  energy,  it  may  be  said  that  the 
majority  of  commercial  heat-treatment  furnaces  involve  the  applica- 
tion to  the  work  of  hot  gases  obtained  through  a  combustion  process. 
If  it  were  then  possible  to  apply  this  heat  so  that  the  charge  would 
be  equally  and  simultaneously  heated  on  all  sides  the  general  ideal 
condition  would  be  met. 

Underfiring. — In  principle,1  it  is  best  to  apply  the  heat  first  to 
the  bottom  of  the  charge.  It  is  natural  law  that  heat  or  hot  gases 
tend  to  rise — a  very  simple  and  important  fact  and  yet  one  so  fre- 
quently overlooked  in  industrial  heat  application.  Further,  since 
in  practical,  every-day  work  the  height  of  suspension  of  the  charge 
(in  order  to  provide  for  circulation  around  the  entire  mass)  is  more 
or  less  reduced  to  a  minimum,  and  which  necessarily  results  in  the 

1  We  say  this,  because  we  will  show  later  that  certain  conditions  may  entirely 
reverse  this  in  practice. — AUTHOR. 


HEAT  APPLICATION 


197 


major  part  of  the  charge  to  be  heated  being  near  the  hearth  or  even 
laying  directly  upon  it,  we  may  consider  that  the  best  construction 
in  general  to  adopt  is  to  place  the  initial  heat  where  it  is  most  needed 
— which  is  under  the  charge.  As  much  advantage  as  possible  is  then 
taken  of  the  natural  law  of  hot  gases  rising  in  effecting  a  further 
application  of  that  heat  to  the  sides  and  top  of  the  charge.  This, 
in  effect,  is  underfiring. 

Simple  Under-fired  Furnace. — A  common  type  of  heat  treatment 
furnace  is  illustrated  in  Fig.  98.  The  heat  is  generated  in  the  com- 
bustion chamber  under  the  hearth,  supplying  heat  to  the  hearth. 
The  hot  gases  then  rise  upon  either  side  of  the  hearth  to  the  roof, 
where  they  become  mixed  or  equalized,  and  then  are  forced  down 
upon  the  floor  of  the  chamber  by  the  pressure  which  has  been  built 


FIG.  98. 


FIG.  99. 


up.  In  the  type  of  furnace  shown  it  will  be  noted  that  the  furnace 
door  opening  is  the  same  height  as  the  roof  arch,  and  that  there  is 
also  a  vent  located  in  the  roof.  Such  a  design  is  permissible  for 
small  furnaces  built  upon  legs  to  lighten  the  weight  for  transpor- 
tation, and  where  first  cost  is  an  important  feature. 

Side  Ledges. — One  of  the  objections  to  this  common  type  of 
furnace  is  the  inherent  tendency  to  overheat  the  sides  of  the  hearth. 
As  the  Tiot  gases  sweep  upwards  from  the  combustion  chamber 
towards  the  roof  the  sides  of  the  hearth  will  become  hotter  than  the 
central  area,  with  the  consequent  superheating  of  any  material 
placed  at  the  sides  of  the  furnace  near  the  ports.  To  overcome  this 
tendency  to  localized  heating,  the  sides  of  the  hearth  are  frequently 
protected  as  illustrated  in  Fig.  99.  Such  an  arrangement  affords 
plenty  of  room  for  circulation  at  the  sides,  tends  to  prevent  cutting 
action  near  the  floor  line  and  automatically  stops  any  overloading 
at  the  sides  of  the  furnace. 


198 


STEEL  AND   ITS  HEAT  TREATMENT 


High  Ledges. — It  was  originally  thought  that  the  flow  and  heat 
transfer  of  the  hot  gas  currents  would  be  in  an  underfired  furnace, 
as  in  Figs.  98  or  99,  from  the  combustion  chamber  to  the  hearth  and 
the  metal  on  the  hearth,  and  thence  to  the  roof.  It  was  then  thought 
that  if  the  hot  gases,  after  leaving  the  combustion  chamber,  could 
be  made  first  to  travel  to  the  roof,  be  thoroughly  equalized,  and 
then  be  brought  down  to  the  hearth,  that  a  more  uniform  heating 
of  the  charge  would  result.  The  furnace  in  Fig.  100  shows  an  early 
development  of  the  underfired  furnace  in  a  misguided  attempt  to 
do  this.  The  side  ledges  were  made  to  reach  nearly  to  the  roof  so 
that  the  gases  had  to  go  there  directly  after  leaving  the  combustion 
chamber.  Experience  with  furnaces  of  the  type  shown  in  Fig.  100 
soon  showed,  however,  that  the  hot  gases — following  natural  law — 


FIG.  100. 


would  first  go  to  the  roof  whether  these  high  side  walls  were  there  or 
not.  In  other  words,  they  were  proven  to  be  not  needed. 

The  second  advantage  hoped  for  through  the  high  ledges  in  Fig. 
100  was  entirely  to  prevent  any  localized  heating  at  the  side  of  the 
charge.  But  it  was  then  discovered  that  the  small  opening  between 
the  top  of  the  ledge  and  the  roof  arch  tended  greatly  to  increase  the 
velocity  of  the  gases  as  they  entered  the  heating  chamber — that  is, 
to  form  a  blast  action  at  the  top  of  the  furnace.  Thus,  if  the  charge 
were  anywhere  near  the  height  of  the  door  opening,  a  localized  heat- 
ing at  the  edges  and  top  of  the  charge  would  at  once  result.  For 
this  and  the  previously  mentioned  reasons  this  type  of  furnace 
(Fig.  100)  is  not  generally  used  now. 

Roof  Vents. — It  will  be  noted  that  in  the  furnaces  in  Figs.  98 
and  99  there  is  a  vent  in  the  rocf  arch,  and  while  this  is  general  prac- 
tice, it  is  not  good.  One  writer,  in  discussing  this  point,  states  that 
"  as  only  approximately  20  per  cent,  of  the  air  for  combustion  is 


HEAT  APPLICATION 


199 


oxygen,  the  balance  is  inert  gases  which  unfortunately  must  be  heated 
to  the  temperature  of  the  furnace  and  expelled  as  quickly  as  possible. 
In  a  scientifically  designed  furnace,  this  is  readily  done  by  the  aid  of 
the  burner.  If  allowed  to  pocket  or  remain  stationary  in  any 
portion  of  the  furnace,  the  inert  gases  cause  uneven  temperatures. 
"  .  .  .  The  vents  for  the  escape  of  ...  the  consumed  and  inert 
gases  should  always  be  located  in  the  oven  roof  or  arch." 

In  the  first  place,  that  writer  evidently  loses  sight  of  the  fact 
that,  with  perfect  combustion,  all  the  gases  become  "  inert  "  (assum- 
ing that  he  means  a  non-supporter  of  combustion);  and  that  the 
furnace  and  stock  are  principally  heated  by  the  blanket  action  of  hot 
gases.  He  admits  that  these  gases  are  hot,  but  then  reasons  that 
because  they  are  hot  and  inert  they  must  be  "  expelled  as  quickly 
as  possible,"  But  why  throw  heat  away?  In  other  words,  he 


FIG.  101. 

believes  in  trying  to  heat  his  house  in  winter  time  by  throwing  open  the 
skylights  or  windows!  He  fails  to  perceive  that  with  an  open  vent 
in  the  roof  the  hot  gas  currents  will  tend  to  short-circuit  directly 
from  the  combustion  chamber  to  the  roof  and  discharge  at  the  max- 
imum chamber  temperature.  (Incidentally,  exactly  what  part  the 
burner — which  is  merely  a  valve  for  injecting  oil  into  a  furnace- 
plays  in  this  ejection  is  not  mentioned.)  But  by  eliminating  the 
vent  in  the  roof  the  gases  are  given  ample  opportunity  thoroughly  to 
mix  in  the  upper  part  of  the  chamber  and  are  then  forced  down  as 
a  blanket  at  a  reduced  velocity  upon  the  stock.  This  same  pres- 
sure, as  we  have  explained  in  the  previous  chapter,  will  prevent  cold 
air  from  the  outside  from  finding  its  way  into  the  furnace.  If 
"  pocketing  "  is  feared  this  may  easily  be  overcome  by  providing 
some  flue  outlet  on  a  level  with  the  hearth,  as  in  Fig.  101. 

Vents  and  Cold  Streaks. — The  question  of  roof  vents  may  also 
be  approached  from  a  different  angle — that  they  will  tend  to  set  up  a 


200 


STEEL  AND   ITS  HEAT  TREATMENT 


current  of  cold  air  through  the  heating  chamber.  The  natural 
tendency  of  roof  vents  is,  as  we  have  previously  said,  to  short-cir- 
cuit the  hot  gases  through  the  roof.  This  means  that  there  will  be 
a  pull  or  suction  around  the  door  towards  the  inside  of  the  furnace, 
across  the  charge  and  thence  to  and  out  through  the  roof.  Cold 
air,  with  free  oxygen,  will,  therefore,  be  sucked  into  the  furnace 
and  will  cause  scaling,  non-uniform  heating  and  widely  variant 
results.  The  further  result  of  these  vents  is  a  loss  of  heat  and  destruc- 
tion of  the  lining  due  to  quick  cooling  after  the  burner  has  been  shut 
off.  This  condition  is  emphasized  by  an  unfortunately  common 
type  of  heating  furnace  shown  in  Fig.  102;  in  this  instance  the  heat 
is  supplied  from  above  the  hearth,  and  by  looking  into  a  furnace  of 
this  type  a  dark  streak  of  cold  air  will  be  seen  to  lay  directly  over  the 
steel1  on  the  hearth. 


FIG.  102. 


Door  Heights.— It  will  also  be  noted  that  in  Figs.  98  and  99  the 
height  of  the  door  opening  is  the  same  as  the  height  of  the  roof  arch ; 
this  is  also  objectionable  practice  under  most  circumstances.  It  is 
reasonable  to  assume  that  the  height  of  the  door  opening  is  governed 
by  the  maximum  height  of  the  charge  desired,  plus  a  reasonable 
allowance  to  facilitate  the  handling  of  the  material.  If  this  is 
true,  then  it  may  be  expected  that  the  height  of  the  charge 
may  frequently  reach  almost  to  the  roof.  This  condition  will  lead 
to  localized  heating  at  the  top  of  the  charge;  and  if  there  is  an  open 
vent  in  the  roof  such  a  condition  will  be  magnified  for  reasons  previ- 
ously mentioned.  It  is,  therefore,  advisable  to  have  the  roof  higher, 
as  illustrated  in  Fig.  103,  so  that  even  with  the  maximum  charge 
there  will  be  ample  space  for  the  gases  to  become  thoroughly  equalized 
and  to  prevent  the  charge  from  encroaching  upon  the  hotter  zone 
close  to  the  roof. 

1  This  particular  example  is  taken  from  a  well-known  spring  shop. 


HEAT  APPLICATION 


201 


The  Heat  Reservoir. — Further,  when  the  door  is  opened  in  the 
furnace  in  Fig.  103,  there  is  always  a  reservoir  of  heat  left  in  the  upper 
part  of  the  furnace  to  aid  in  heating  the  next  charge  and  maintaining 
the  temperature  of  the  furnace  during  discharging  or  recharging 


FIG.  103. 


FIG.  104. 


FIG.  105. 

operations.  With  the  furnaces  hi  Figs.  98  and  99  this  is  impossible, 
for  when  the  door  is  opened  the  furnace  will  be  almost  entirely 
emptied  of  its  hot  gases  because  the  door  opening  is  the  same  height 
as  the  roof.  This  is  perhaps  more  clearly  illustrated  by  the  furnaces 
in  Figs.  104  and  105.  It  is  better  practice  to  have  the  roof  higher 


202 


STEEL  AND   ITS  HEAT  TREATMENT 


than  the  door  opening,  and  always  to  keep  the  door  height  as  low 
as  possible,  as  is  illustrated  by  the  furnace  in  Fig.  106. 

Height  of  Chamber  vs.  Height  of  Charge. — In  order  to  make 
quick  use  of  the  heat  which  is  thus  retained  in  the  furnace,  it  is  a 
part  of  the  furnace  man's  job  to  provide  for  ample  space  for  circula- 
tion throughout  the  mass.  Thus  with  the  charge  arranged  as  in 
Fig.  104  there  is  ample  room  for  the  circulation  of  the  hot  gases 
around  the  charge,  and  the  heat  application  would  be  much  better 
and  more  uniform  than  in  Fig.  105,  where  the  top  of  the  charge  is 
near  the  roof.  From  this  it  will  be  seen  that  the  size  and  arrange- 
ment of  the  charge  is  an  indication  of  the  heat  application  value 
of  a  furnace.  Better  results  will  be  obtained  in  a  high  chamber 
with  a  low  charge  than  if  the  charge  is  higher. 


FIG.  106. 


It  is  better  practice  to  have  the  chamber  high  enough  to  afford 
plenty  of  space  for  circulation,  as  is  illustrated  in  Fig.  106.  This 
diagram  also  illustrates  the  points  previously  made  concerning  roof 
vents  and  door  heights. 

Influence  of  Mass. — The  furnaces  shown  in  Figs.  107  to  115  incl. 
illustrate  different  commercial  methods  for  the  pot  annealing  of 
wire,  and  are  particularly  a  propos  as  examples  of  heat  application 
on  account  of  the  size  of  the  mass  to  be  heated. 

Thus  in  Fig.  107,  we  have  a  furnace  with  a  combustion  chamber 
under  the  hearth,  and  with  a  large  pot  resting  directly  upon  the 
hearth.  No  matter  how  uniform  the  temperature  may  be  in  the 
chamber  there  will  always  be  a  tendency  for  a  cold  zone  to  form  at 
the  bottom  of  the  pot — as  is  represented  by  the  shaded  portion  in  the 
drawing.  This  is  due  to  the  fact  that  the  floor  of  the  furnace  is  of  a 
refractory  nature,  and  will  not  transmit  the  heat  as  fast  as  the  pot 


HEAT  APPLICATION 


203 


will  take  it  away.  The  placing  of  the  pots  in  the  furnaces  in 
Figs.  109,  110  and  111  is  better  in  this  respect,  inasmuch  as  it  pro- 
vides for  the  circulation  of  the  hot  gases  under  the  pot. 


FIG.  107. 


FIG.  108. 


With  a  short  pot,  as  in  Fig.  107,  it  is  sometimes  permissible  to 
use  a  furnace  in  which  the  heat  is  applied  from  the  top  of  the  charge 
downwards;  but  there  is  a  limit  to  this  because  when  the  height  of 


I 

T 

iip^l           t^y 

V///////////////A 

FIG.  109. 


FIG.   110. 


the  pot  increases,  as  with  Figs.  108,  110  and  111,  there  will  be  a 
tendency  to  overheat  the  top  of  the  pot  before  the  bottom  is  at  the 
right  temperature.  For  this  reason  the  height  of  the  pot  (or  charge) 
in  itself  determines  the  type  of  furnace. 


204 


STEEL  AND   ITS  HEAT   TREATMENT 


In  some  plants  the  heat  is  applied  to  the  bottom  (but  without  a 
separate  combustion  chamber)  and  taken  off  at  the  top,  as  in  Fig. 
110;  but  even  with  this  construction  it  is  difficult  to  secure  a  uni- 
formly heated  product,  as  the  temperature  of  the  top  and  bottom  of 
the  furnace  rarely  would  be  the  same.  In  this  respect  the  design  of 
Fig.  Ill  is  better.  This  latter  provides  for  underfiring,  for  circula- 
tion under  the  pot,  and  further  takes  the  gases  off  at  the  hearth 
(not  shown) ,  inasmuch  as  the  heat  must  rise  to  the  roof  and  return 
to  the  floor  before  it  can  escape. 

When  the  construction  in  Fig.  110  (with  a  heavy  roof -door) 
takes  the  form  of  that  in  Fig.  112  (with  a  cast-iron  top),  as  it  fre- 


FIG.  ill. 


FIG.  112. 


quently  does  in  practice,  the  method  is  open  to  still  more  criticism. 
In  either  case  the  removal  of  the  door,  which  constitutes  the  roof 
of  the  furnace,  permits  the  escape  of  a  maximum  amount  of  heat 
and  cools  off  the  furnace.  But  in  Fig.  112  the  roof,  being  of  metal, 
radiates  a  great  amount  of  heat  even  with  the  top  on  the  furnace, 
and  is  severe  on  the  men.  It  is  better  practice  to  employ  a  type  of 
furnace  in  which  the  charge  is  introduced  through  a  door  or  opening 
in  the  side  or  end  instead  of  through  the  top  of  the  furnace. 

Fig.  113  illustrates  a  construction  of  pot  that  is  employed  very 
successfully  in  annealing  wire,  and  it  gives  very  good  results,  inas- 
much as  the  heat  is  applied  to  the  center  of  the  coils  as  well  as  the 


HEAT  APPLICATION 


205 


outside,  and  to  the  bottom  as  well  as  the  top.     It  goes  to  prove  that 
the  type  of  charge  has  much  to  do  with  uniform  heating. 

Figs.  114  and  115  illustrate  the  relative  merits  of  heating  high 
pots  from  the  bottom  up  (Fig.  114)  and  from  the  top  down  (Fig. 
115).  In  the  underfired  furnace,  also  provided  with  a  movable 
ball  type  carriage  or  charging  device,  the  heat  is  again  first  applied 
where  it  is  most  needed — at  the  bottom — and  the  hot  gases  rising 
will  naturally  take  care  of  the  heat  application  at  the  top.  In  the 
overfired  furnace  (Fig.  115)  the  gases  must  descend  and  even  though 


FIG.  113. 


FIG.  114, 


the  heat  js  forced  to  circulate  under  the  hearth,  that  part  of  the  hearth 
under  the  charge  will  always  be  the  cold  spot,  due  to  the  fact  that  the 
absorptive  powers  of  the  iron  pot  is  greater  than  the  heat  input  of  the 
hearth  by  the  hot  gases  thereunder.  The  relative  advantages  of  the 
two  methods  of  heat  application  will  be  more  fully  discussed  in  sub- 
sequent sections,  but  it  is  here  evident  that  the  height  of  charge 
again  determines  the  method  of  applying  the  heat. 

Influence  of  Character  of  the  Charge. — A  charge  in  a  furnace 
heats  up  in  about  the  same  manner  as  a  plate  of  ice  cream  while 
melting— that  is,  from  the  top  and  outside  edges  towards  the  center — 


206 


STEEL  AND  ITS  HEAT  TREATMENT 


particularly  when  the  heat  is  applied  from  above.  The  difference 
between  uniform  chamber  temperature  and  uniformly  heated  product 
is  illustrated  by  Figs.  116  and  117,  in  which  it  is  assumed  that  the 


FIG.  115. 

charge  consists  of  small  pieces — such  as  lock-washers  or  cartridge 
shells — laid  on  the  chamber  floor.  It  is  immaterial  in  this  case 
whether  the  furnace  is  underfired  (Fig.  116)  or  overfired  (Fig.  117), 


FIG.  116. 

or  how  uniform  the  actual  temperature  in  the  chamber  may  be, 
because  there  will  always  be  a  tendency  for  a  cool  section  in  the 
center,  as  shown  in  Fig.  118.  It  is  practically  impossible  to  get  a 


HEAT  APPLICATION 


207 


uniformly  heated  product  under  such  conditions  unless  the  charge 
is  so  split  up  as  to  permit  free  circulation  of  heat  through  the  mass. 
The  real  test  in  annealing,  hardening  or  tempering  is  in  the  uni- 
formity of  cooling — that  is  to  say,  it  is  not  only  necessary  that  a  piece 
should  be  uniformly  heated,  but  it  must  also  be  uniformly  cooled. 
For  this  reason,  instead  of  heating  small  pieces  as  shown  in  Figs. 
116  and  117,  it  is  better  practice  to  employ  an  automatic  type  of 


FIG.  117. 

furnace  like  Figs.  119  or  121.  With  these  designs  each  surface 
of  each  piece  is  exposed  to  the  action  of  the  heat,  and  it  is  heated 
and  cooled  uniformly.  In  other  words,  the  material  to  be  heated  is 
indicative  of  the  method  of  heat  application  and  furnace  design. 

Muffle  Furnaces. — Muffles,  as    ordinarily  constructed  for  heat 
treatment  work,  do  not  necessarily  prevent  oxidation,  other  claims 


FIG.  118. 

to  the  contrary.  The  following  are  statements  taken  from  recent 
publications  on  this  subject:  (a)  "  \Vhenoxidation  or  the  formation 
of  scale  is  particularly  objectionable,  furnaces  of  the  muffle  type 
are  often  used,  having  a  refractory  retort  in  which  the  steel  is 
placed  so  as  to  exclude  the  products  of  combustion."  (6)  "  The 
metal  does  not  become  saturated  with  any  of  the  products  of 
combustion  .  .  .",  referring  to  the  furnace  illustrated  in  Fig.  123. 


208 


STEEL  AND  ITS  HEAT  TREATMENT 


The  first  statement,  referring  to  the  merits  of  the  muffle  versus 
the  open  chamber,  must  lead  to  the  conclusion  that  muffles  are  synon- 
ymous with  improper  open  chamber  work.  We  have  previously 
explained  that,  with  proper  furnace  design  and  correct  operation, 
an  atmosphere  may  be  produced  which  will  contain  no  free  oxygen, 


FIG.  119. 


FIG.  120. 


no  oil  vapors,  and  just  enough  carbon  monoxide  to  take  care  of  any 
air  which  may  possibly  find  its  way  into  the  heating  chamber  through 
unforeseen  causes ;  that  this  slightly  hazy  atmosphere  results  from  an 
absolute  control  of  the  air  supply,  in  combination  with  the  right 
furnace  design;  and  that  a  furnace  operated  in  such  a  manner  will 


FIG.  121. 


FIG.  122. 


give  a  product  which  will  often  be  better  than  that  heated  under 
charcoal.  Thus  when  it  is  found  necessary  to  exclude  the  products 
of  combustion  from  contact  with  the  hot  steel,  it  means  that  such 
gases  contain  free  oxygen,  and  which  is  identical  with  improper 
furnace  operation  or  design. 


HEAT  APPLICATION 


209 


Again,  if  the  products  of  combustion  are  excluded  from  the  muffle, 
the  question  reverts  to  the  fact  that  there  is  no  method  of  keeping 
the  outside  air  or  oxygen  from  finding  its  way  into  the  muffle.  As 
there  is  no  pressure  of  gases  from  within,  since  the  pressure  which 
should  be  caused  by  the  products  of  combustion  is  absent,  the  free 
oxygen  must  inevitably  find  its  way  into  the  muffle.  For  these 
reasons,  therefore,  muffles  do  not  prevent  oxidation. 

Semi-Muffle  Furnaces. — On  the  other  hand,  if  a  pressure  is  built 
up  from  within  the  muffle  to  prevent  air  from  entering,  there  must 
be  some  opening  between  the  muffle  and  the  hot  gas  chamber  sur- 
rounding it.  In  such  a  case  it  is  no  longer  a  true  muffle  and  does  not 
exclude  the  products  of  combustion.  Thus  in  Fig.  123  there  are 


FIG.  123, 


shown  openings  in  the  roof  of  the  muffle.  If  the  course  of  the  hot 
gases  is  such  that  the  gases  come  downwards  through  these  openings, 
then  why  have  any  roof  at  all  on  the  muffle? — for  the  products  of 
combustion  enter  the  heating  chamber  and  the  furnace  approximates 
open  chamber  construction.  Or,  if  the  flow  is  upwards  through  these 
openings,  then  outside  air  will  be  sucked  into  and  through  the  muffle, 
and  oxidation  will  be  set  up  in  that  manner.  In  other  words,  whether 
or  not  the  furnace,  as,  and  for  the  purpose  designed,  is  properly 
operated,  there  is  no  occasion  for  the  remaining  solid  part  of  the 
muffle  roof,  and  the  statement  contradicts  itself. 

Influence  of  Nature  of  Fuel  on  Furnace  Design. — The  overfired, 
perforated-arch  type  of  furnace — under  which  Fig.  123,  previously 
discussed,  may  be  classed,  and  which  general  type  is  common  to  the 
construction  shown  in  Fig.  124 — is  a  development  originally  intended 


210 


STEEL   AND   ITS  HEAT  TREATMENT 


to  meet  certain  conditions  with  oil  fuel,  but  which  is  neither  necessary 
nor  desirable  with  gas  or  coal.  The  development  also  illustrates  the 
difference  between  uniform  application  of  heat  and  uniform  applica- 
tion of  fuel  which  was  discussed  in  the  last  chapter  (q.v.). 

With  a  type  of  rotary  annealing  furnace  like  that  in  Figs.  121 
and  122,  using  gas  as  a  fuel,  the  burners  are  usually  placed  on 
both  sides,  so  that  a  small  amount  of  fuel  is  injected  through  each 
burner.  This  distributes  the  application  of  the  fuel.  With  oil, 
however,  the  consumption  under  a  similar  arrangement  of  burners 


FIG.  124. 


would  be  so  low  that  the  burners  could  not  be  kept  going  steadily, 
and  for  this  reason  it  was  desired  to  employ  but  one  burner.  This, 
in  turn,  was  open  to  the  objection  that  there  would  be  a  hot  streak 
directly  in  front  of  the  burner  and  which  would  react  unfavorably  on 
the  charge.  To  overcome  this  the  perforated  arch  was  employed  so 
as  to  lessen  the  streaking  effect  of  the  flame  and  thus  distribute  the 
heat,  as  is  shown  in  Figs.  119  and  120. 

It  is  therefore  evident  that  the  nature  of  the  fuel  in  these  cases 
determines  the  design  of  the  furnace.  In  the  first  case  (Figs.  121 
and  122)  the  uniformity  of  heating,  aside  from  the  nature  of  the 


HEAT  APPLICATION 


charge,  is  secured  by  a  uniform  burn- 
ing of  the  fuel — gas — throughout  the 
length  of  the  furnace.  In  the  second 
case  (Figs.  119  and  120)  the  fuel- 
oil — input  is  concentrated  and  the 
perforated  arch  construction  is  em- 
ployed to  secure  the  heat  distribution. 
The  perforated  arch,  which  was  found 
advisable  in  the  case  of  oil,  for  the 
purpose  in  view,  is  entirely  unnecessary 
with  gas  fuel. 

Another  illustration  of  this  per- 
forated-arch type  as  applied  to  a 
long,  low  hearth  with  concentrated 


FIG.  125. 

fuel  supply,  and  which  followed  the 
above  development,  is  shown  in  Figs. 
125  and  126.  In  this  case  also  the 
fuel  input  is  from  one  side  of  the  fur- 
nace and  above  the  hearth;  the  per- 
forated ,  arch  distributes  the  heat  to 
the  chamber  beneath  and  from  which 
the  gases  pass  underneath  the  hearth. 
But  it  should  be  remembered  that 
while  the  perforated  arch  construc- 
tion may  be  perfectly  proper  under 
certain  conditions,  it  may  be  en- 
tirely out  of  place  under  other 
conditions,  and  yet  using  the  same 
fuel. 


O 


o 


O 


212 


STEEL  AND   ITS  HEAT  TREATMENT 


Perforated-arch  Furnaces. — From  the  type  of  furnace  of  Figs. 
125  and  126  it  is  but  a  short  step  to  the  overfired,  perforated-arch 
furnace  shown  in  Fig.  124  (previously  alluded  to),  and  which  has 
been  somewhat  widely  employed  for  general  heat  treatment  work — 
often  regardless  of  distinctive  shop  and  heat  application  conditions. 
It  will  be  noted  that  there  are  burners  on  both  sides  of  the  furnace 
(generally  staggered),  that  the  combustion  takes  place  in  a  cham- 
ber above  the  main  wyorking  chamber,  and  that  the  gases  then  pass 
down  through  the  perforated  arch  into  that  chamber  and  are  taken 
out  from  under  the  floor. 


FIG.   127. — De-carburization  of  Steel  by  High-velocity  Gases. 
X60.      (Bullens.) 

This  design  provides,  in  effect,  for  the  application  of  heat  from 
above  through  a  perforated  arch,  and  is  permissible  with  low  charges, 
but  when  the  charges  are  high  there  is  a  tendency  to  overheat  the 
top.  If  a  thorough  study  is  made  of  the  perforated  arch  itself, 
it  will  be  found  that  the  actual  openings  only  total  about  twenty- 
five  or  thirty  per  cent,  of  the  total  chamber  area.  With  a  continual 
input  of  fuel  and  air  into  the  hot  combustion  chamber  above,  and 
with  but  a  small  exit  for  the  hot  gases,  these  hot  gases  must  enter 
the  working  chamber  at  a  high  velocity.  If  the  furnace  is  charged 
to  anywhere  near  the  height  of  the  working  opening  or  arch,  which 


HEAT  APPLICATION  213 

is  a  common  procedure,  the  hot  gases  will  impinge  upon  the  top  of 
the  charge  at  high  velocity.  This  inevitably  results  in  severe  cut- 
ting action  and  oxidation,  the  zone  of  which  is  shown  in  the  charge 
in  Fig.  124.  This  is  further  illustrated  by  the  photomicrograph 
of  Fig.  127,  taken  from  the  edge  of  an  annealing  charge  of  chrome 
nickel  steel  plates  piled  as  illustrated  in  Fig.  124.  It  will  be  seen 
that  the  steel  has  been  entirely  decarburized  along  one  edge,  even 
though  the  actual  indicated  temperature  of  the  furnace  was  only 
about  1350°  F.  In  the  plant  from  which  this  example  was  taken  it 
was  no  uncommon  occurrence  to  lose  as  much  as  J  or  even  J  in.  of 
metal  on  each  side  of  a  pile  10  or  12  in.  wide. 

Aside  from  changing  the  type  design  of  the  furnace,  the  only 
method  of  overcoming  this  particular  trouble  without  decreasing 
the  production  is  to  increase  the  height  of  the  working  chamber. 
In  this  manner  the  gases  are  given  an  opportunity  to  expand  before 
reaching  the  metal,  and  thus  reduce  the  high  velocity  caused  by  the 
perforated  arch.  In  other  words,  an  overfired,  perforated-arch 
furnace  is  permissible  with  low  charges,  in  comparison  with  cham- 
ber height,  such  as  is  shown  by  the  dotted  line  charge  in  Fig.  124. 

Overfired  Furnaces. — In  order  to  overcome  the  necessity  for 
comparatively  high-working  chambers,  as  occasioned  by  the  con- 
ditions above  referred  to,  the  perforated-arch  construction  may  be 
eliminated.  This  results  in  a  type  of  overfired  furnace  illustrated 
in  Fig.  128.  In  this  case  the  charge  is  heated  from  the  top  down- 
wards, and  the  gases  pass  out  from  under  the  floor.  Aside  from 
the  fact  that  the  hot  gases  have  to  pass  downwards,  the  question 
then  arises  as  to  the  manner  in  which  the  charge  is  heated.  Since 
the  heat  is  applied  from  above  there  is  no  question  but  that  the 
top  of  the  charge  will  be  heated ;  but  how  about  the  bottom  of  the 
charge? 

It  should  be  borne  in  mind  that  this  construction  (with  flues 
under  the  hearth)  does  not  necessarily  result  in  a  hot  floor,  even 
though  it  be  granted,  for  sake  of  argument,  that  there  is  a  consider- 
able volume  of  gases  under  the  floor  and  that  the  temperature  of 
these  gases  is  the  same  as  above  the  floor.  The  specific  heat  of  the 
charge  is  such  that  it  absorbs  heat  from  the  floor,  and  unless  the  rate 
of  input  is  greater  than  the  rate  of  absorption  the  floor  will  cool  under 
the  charge  in  proportion  to  the  manner  in  which  it  is  packed.  This 
cold  spot,  in  turn,  means  a  cold  zone  through  the  center  and  bottom 
of  the  charge,  and  until  this  cold  zone  is  removed  the  charge  is  not 
uniformly  heated.  But  in  order  to  remove  it,  it  is  necessary  to 


214 


STEEL  AND   ITS  HEAT  TREATMENT 


lengthen  the  time  of  exposure,  which  results  in  a  tendency  to  expose 
the  outside  edges  of  the  charge  to  the  action  of  the  heat  and  gases 
longer  than  is  necessary  with  other  construction.  In  practice  this 
cold  spot  is  never  totally  eliminated  in  this  type  of  furnace. 

Even  with  the  I-bar  floor  construction  illustrated  in  Fig.  128, 
which  is  the  best  in  use  for  this  type  of  furnace,  the  floor  is  not  as. 
hot  as  it  should  be.  The  reason  for  this  is  that,  irrespective  of 
the  temperature  or  volume  of  heat  under  the  floor,  the  rate  of  trans- 
mission of  heat  to  the  under  side  of  the  charge  is  no  greater  than 
that  possible  through  the  vertical  section  of  the  I-bar.  The  rate 
of  transmission  through  the  tiles  separating  the  bars  is  still  less  than 


FIG,  128. 


through  the  bars  themselves  on  account  of  the  low  conductivity 
of  the  material.  When  the  construction  is  made  without  the 
I-bars,  as  it  sometimes  is  to  lower  the  cost,  the  conditions  are  still 
worse. 

From  this  it  will  be  seen,  as  has  been  repeatedly  proven  in 
practice,  that  there  is  a  disadvantage  in  any  construction  which 
does  make  possible  a  floor  temperature  equal  to  that  above  the 
charge;  or  a  circulation  of  heat  under  the  charge  to  make  up  the  loss 
in  transmission  and  decrease  the  time  of  exposure  by  decreasing 
the  area  of  the  cold  zone. 

Influence  of  Arrangement  of  Charge. — Such  a  condition  just 
described  illustrates  very  forcibly  the  difference  in  heat  application 
obtained  when  a  furnace  is  full  and  when  it  is  empty.  That  a 


HEAT  APPLICATION  215 

furnace  will  give  uniform  temperatures  without  a  charge  is  no  cri- 
terion that  the  heat  application  to  a  charge  will  be  uniform.  The 
type  of  furnace  illustrated  in  Figs.  129  to  134  is  used  extensively 
in  the  annealing  or  wire  and  tool  steel,  and  many  people  wonder 
why  it  is  that  with  almost  perfect  pyrometer  records  they  do  not  get 
a  uniform  product. 

The  furnace  is  fired  with  coal  from  a  fire-box  at  one  end,  the  flame 
and  heat  passing  over  a  bridge  wall  to  the  heating  chamber;  at  the 
other  end  of  the  hearth  the  hot  gases  pass  down  and  under  the 
hearth  through  a  series  of  flues,  and  from  thence  to  the  chimney. 
The  hottest  part  of  the  hearth  is  near  the  bridge  wall. 

In  the  case  of  the  charge  of  wire  in  Figs.  129  and  130  the  non- 
uniformity  of  product  is  partly  due  to  the  fact  that  the  first  piece 
in  is  the  last  piece  out  and  vice  versa,  so  that  the  first  piece  is  exposed 
to  the  highest  temperature  for  the  longest  time,  and  the  last  piece 
to  the  lowest  temperature  for  the  shortest  time. 

With  the  method  of  charging  tool  steel  in  Figs.  131  and  132  the 
non-uniformity  is  partly  due  to  the  lack  of  circulation  through  the 
charge;  the  tubes  rest  directly  upon  the  hearth  and  are  packed 
tightly  together.  This  can  be  somewhat  overcome  by  rearranging 
the  tubes  as  in  Figs.  133  and  134,  separating  them  and  raising  them 
up  from  the  hearth. 

All  of  these  (Figs.  129-134)  are  open  to  the  objection  that  the 
heat  is  not  uniform  throughout  the  length  of  the  charge,  and  while 
it  is  possible  to  vary  the  quality  of  the  product  by  rearranging  the 
charge  without  affecting  the  pyrometer  readings,  as  above  illus- 
trated, there  is  still  the  fact  that  the  heat  should  be  uniformly 
applied  throughout  the  entire  length  and  through  the  mass  in  order 
to  secure  a  uniformly  heated  product. 

This  latter  point  is  also  illustrated  by  Figs.  135  and  136.  In 
carburizing  work  the  practice  of  Fig.  135  is  often  followed,  filling  the 
furnace  to  its  maximum  capacity  by  packing  the  boxes  up  to  the  side 
walls  as  well  as  in  front.  It  is  much  better  practice  to  maintain 
circulating  space  on  the  sides  and  ends,  as  shown  by  Fig.  136,  and 
not  to  place  the  boxes  beyond  a  certain  imaginary  line  such  as  is 
illustrated  by  the  dotted  line  of  the  drawing.  The  method  of 
handling  or  arranging  the  charge  in  the  furnace  is  equally  important 
with  correct  furnace  design  and  proper  operation  or  heat  distribution. 

Other  Furnace  Designs. — The  designs  of  furnaces  intermediate 
between  the  two  principal  types — underfired  and  overfired — are 
legion.  One  characteristic  furnace  in  common  use  is  that  illustrated 


216 


STEEL  AND  ITS  HEAT  TREATMENT 


C 


HEAT  APPLICATION 


217 


by  Fig.  137.  In  this  it  will  be  noted  that  the  underfiring  principle 
has  been  used,  locating  the  combustion  chamber  under  the  hearth; 
that  the  hot  gases  pass  upwards  to  the  heating  chamber  on  one 


FIG.  135. 


FIG.  136. 


FIG.  137. 


side  of  the  hearth;  and  that  a  roof  vent  is  located  on  the  opposite 
side  of  the  chamber.  In  this  design  the  benefit  of  underfiring  is 
largely  negatived  by  the  poor  heat  application  to  that  part  of  the 
charge  and  hearth  directly  under  the  vent  and  farthest  removed 


218 


STEEL  AND   ITS  HEAT  TREATMENT 


from  the  heating  chamber  intake  ports;  the  hot  gas  currents  will 
short-circuit  from  the  ports  to  the  vent.  A  good  overtired  furnace 
would  be  better  practice. 

Coal  Furnaces. — Fig.  138  illustrates  a  common  type  of  heat  treat- 
ment furnace  using  hard  coal  as  the  fuel.  The  heat  is  generated 
from  coal  placed  on  the  grate  at  the  left  of  the  furnace  illustrated, 
passes  over  the  bridge  wall  into  the  heating  chamber,  and  then  under 
the  hearth  to  the  flues  and  to  the  chimney.  From  points  previously 
raised  upon  other  furnaces  there  will  be  noted  the  tendency  to 
localized  heating  near  the  bridge  wall,  the  fact  that  the  height  of 


FIG.  138. 

the  door  opening  is  virtually  the  same  as  that  of  the  roof  arch, 
and  the  tendency  towards  a  cold  hearth.  In  such  coal  furnaces  as 
generally  designed  and  operated  there  is  a  decided  lack  of  control 
of  the  volume,  temperature  and  composition  of  the  gases  to  and  from 
all  points  in  the  chamber. 

Again,  when  local  conditions  advocate  the  use  of  a  cheap  fuel, 
such  as  coal  (and  why  use  hard  coal  at  $7  or  so  a  ton  when  soft 
coal  at  $2  a  ton  could  be  made  to  do  the  same  work?),  the  gener  1 
method  of  burning  it,  as  here  illustrated,  is  found  to  be  inefficient 
from  the  standpoint  of  fuel  application  and  unsatisfactory  from 
the  heat  application  viewpoint.  The  only  proper  way  to  attack 
such  a  problem  is  first  to  gasify  the  coal  and  properly  to  utilize  that 
gas;  not  necessarily  to  generate  the  gas  in  a  separate  producer 


HEAT  APPLICATION 


219 


and  carry  it  by  expensive  flues  to  the  furnace,  but  to  combine  the 
two  operations  in  one  efficient,  self-contained  unit.  Such  is  being 
done,  and  such  furnaces  are  today  producing  a  better  heated 
product,  at  less  operating  cost,  than  many  furnaces  using  oil  or  gas. 


FIG.  189. 


FIG.  140. 


Car-bottoms.— Figs.  139,  140  and  141  represent  three  designs  of 
the  car  type  furnace — the  open-chamber,  perforated-arch  and  coal- 
fired  furnaces  respectively. 

Furnaces  with  the  movable  car-bottom  may  be  mechanically 


220 


STEEL  AND   ITS  HEAT  TREATMENT 


efficient,  but  they  are  thermally  inefficient.  In  cases  where  there  is 
much  work  to  be  handled  of  a  large  and  variable  size,  the  use  of 
such  furnaces  may  be  advisable.  But  it  is  the  same  proposition  of 
cold  hearths  versus  hot  hearths  which  has  been  previously  discussed, 
but  carries  the  matter  one  step  farther.  In  this  case,  each  time 
the  hot  car  is  removed  from  the  furnace  a  large  amount  of  heat 
is  lost,  and  the  furnace  must  be  refired  with  a  cold  hearth.  Simi- 
larly, the  radiation  losses  during  the  time  between  the  removal  of 
one  charge  and  the  placement  of  the  new  car  are  very  great.  How- 
ever, the  saving  effected  in  both  labor  and  time  may  be  consider- 
able under  certain  conditions  as  above  noted,  but  in  any  event 


FIG.  141 


the  charge  should  be  raised  from  off  the  hearth  lo  give  the  best 
circulation  possible.  Furnaces  with  car-bottoms  should  only  be 
used  when  there  are  no  others  which  will  prove  as  commercially 
effective. 

Underfired  Furnaces. — The  aim  of  any  uniform  heating  opera- 
tion should  be  to  supply  heat  to  all  sides  of  the  charge.  Under 
ordinary  conditions,  this  is  probably  most  nearly  accomplished 
in  the  underfired  type  of  furnace  with  a  perforated  floor,  and  an 
effort  has  been  made  to  overcome  the  disadvantages  previously 
referred  to  in  the  discussion  of  other  types  of  furnaces.  Fig.  142 
illustrates  a  recent  patented  type  of  underfired  furnace,  and  com- 
parison should  be  made  with  the  overfired  furnaces  previously 
described,  such  as  in  Fig.  128. 


HEAT  APPLICATION 


221 


The  heating  is  done  from  the  bottom  upwards  instead  of  from 
the  top  downwards.  Heat  naturally  rises,  and  with  such  construc- 
tion as  in  Fig.  142,  if  the  floor  is  hot  the  roof  is  hot,  although  it  is 
possible  to  obtain  the  reverse  in  an  overfired  furnace.  Even 
in  an  underfired  furnace  the  bottom  can  never  be  heated  more  than 
the  top.  In  the  construction  outlined  in  the  drawing,  the  gases 
are  passed  through  large  combustion  chambers  and  compelled  to 
circulate  through  several  hundred  feet  of  ports  on  both  sides,  as 
well  as  through  the  floor,  each  of  which  exposes  considerable  area 


FIG.  142. 

against  which  the  gases  are  wiped.  In  this  way  there  is  a  minute 
subdivision  of  the  volume  and  a  thorough  mixture. 

Such,  a  furnace  design  is  also  in  accord  with  the  fact  that  correct 
heating  is  a  function  of  pressure.  Thus  the  heat,  in  going  to  the 
roof,  naturally  stratifies  and  builds  up  a  natural  pressure,  which 
will  be  spread  over  the  entire  area  before  it  will  come  down.  The 
result  is  a  pressure  always  on  the  floor  and  the  elimination  of  streams 
of  gases  of  unequal  temperature  and  composition.  The  hot  gases 
surround  the  steel  as  a  blanket,  and  have  a  minimum  velocity. 
With  such  methods  of  applying  the  heat,  the  area  of  the  cold  zone  is 
quickly  decreased,  and  this,  in  turn,  lessens  the  length  of  exposure. 

It  will  also  be  noted  that  provision  is  made  for  circulation  on 


222  STEEL  AND   ITS  HEAT  TREATMENT 

both  sides  of  the  charge,  independent  of  the  manner  in  which  it  is 
packed.  It  is  impossible  to  overload  the  furnace  and  cut  off  the 
circulation,  and  even  though  the  charge  were  the  full  width  and 
height  of  the  working  opening  there  would  still  be  room  on  each 
side  for  circulation.  In  the  particular  furnace  illustrated,  this 
extra  room  costs  about  three  feet  in  chamber  width;  the  area  for 
circulation  on  the  sides  is  about  30  per  cent,  of  the  width  of  door, 
and  it  is  by  the  room  afforded  with  such  greater  width  that  the 
velocity  of  the  gases  is  cut  down. 

On  the  other  hand,  and  in  the  case  of  the  overfired  furnace  of 
Fig.  128,  it  will  be  noted  that  if  the  chamber  is  the  same  width 
as  the  working  opening,  it  will  be  necessary  to  employ  compara- 
tively small  charges  in  order  to  get  circulation  through  the  restricted 
areas  on  the  sides.  To  gain  time,  with  this  practice  there  is  a  dan- 
ger of  overheating  for  the  reason  that,  as  the  area  is  decreased,  the 
pressure  must  be  increased  for  a  given  B.T.U.  input,  which  works 
out  in  practice,  as  a  rule,  to  the  detriment  of  the  top  and  exposed 
edges  of  the  charge. 

Flue  Construction. — For  furnaces  of  any  considerable  size,  flue 
construction  is  absolutely  necessary,  not  only  to  provide  an  escape 
for  the  waste  gases,  but  also  to  direct  the  hot  gas  currents  during 
their  passage  through  the  furnace,  and  to  conserve  the  heat  in  those 
gases  after  they  have  left  the  heating  chamber  proper.  Primarily, 
the  practice  is  to  circulate  the  gases  around  the  stock  to  be  heated, 
to  heat  the  chamber  as  a  whole,  and  then  to  pass  them  out  at  the 
coldest  part  of  the  furnace.  Some  of  the  furnace  drawings  given 
have  shown  in  some  degree  such  provisions,  but  in  order  not  to  com- 
plicate the  discussion  of  heat  application,  the  subject  of  heat  con- 
servation has  been  little  dwelt  upon.  The  latter  is,  in  fact,  a  prob- 
lem which  must  be  studied  out  for  each  particular  design.  In  any 
case  the  flues  should  be  arranged  so  as  to  prevent  short-circuiting 
of  the  hot  gas  cycle. 

Conservation  of  Heat. — Additional  flue  construction  and  thicker 
walls  both  tend  towards  the  conservation  of  waste  heat.  The  dis- 
cussion thus  far  has  had  to  do  with  single  furnaces,  and  the  extent 
to  which  the  thickness  of  the  walls  might  be  increased  is  obviously 
restricted  within  narrow  limits  by  the  cost  of  construction.  Since 
losses  by  radiation  are  largely  preventive,  any  arrangement  or 
grouping  together  of  furnaces  of  a  similar  type  which  will  tend  to 
unite  them,  thus  eliminating  exposed  walls,  should  be  made  a  sub- 
ject of  study. 


HEAT  APPLICATION 


223 


Variety  of  Furnace  Plans. — The  diagrams  in  Figs.  143, 144  and  145 
are  intended  to  illustrate  as  floor  plans  a  few  of  the  many  different 


furnace  designs  which  are  employed  in  practice.     All  of  these,  which 
are  more  or  less  empirical,  as  well  as  hundreds  of  others  not  shown, 


224 


STEEL  AND   ITS  HEAT  TREATMENT 


have  been  built  in  a  variety  of  sizes  for  oil,  gas,  coal,  coke  and  wood, 
with  different  methods  of  applying  the  heat  to  suit  different  opera- 
tions ranging  from  small  needles  to  eighty  tons  of  steel  at  a 
charge. 

In  designing  furnace  equipment  it  is  not  only  necessary  to 
consider  combustion  and  the  more  important  points  of  heat  applica- 
tion as  well  as  the  fuel  suited  to  both,  but  likewise  the  method  of 
handling  material  to  and  from  the  furnace,  together  with  the  floor 


WA    w/, 


y////////////////, 


V77A 


V//7A      W/, 


FIG.  144. — Unit  Furnace  System  Development. 


space  available,  which  are  no  small  factors  in  the  cost  of  production 
and  installation. 

The  purpose  should  be  to  keep  the  material,  the  men,  the  fur- 
naces and  machinery  in  continuous  operation,  or  as  near  it  as  possi- 
ble, because  each  is  more  or  less  dependent  upon  the  others  and 
all  must  be  properly  linked  together  to  secure  the  best  all-around 
results.  It  is  just  as  necessary  to  adapt  the  furnace  design  to  manu- 
facturing conditions  as  it  is  with  machine  tools,  but  the  latitude 
for  variation  is  much  greater. 

Owing  to  the  great  variety  of  heating  operations  and  shop 
conditions,  it  is  rarely  found  that  the  same  identical  furnace  can  be 


HEAT  APPLICATION 


225 


properly  employed  in  two  shops,  or  even  in  separate  departments 
of  the  same  shop,  for  similar  operations. 

Thus  these  sketches  will  serve  to  illustrate  some  of  the  develop- 
ment that  has  been  made,  as  well  as  the  latitude  possible  in  design- 
ing furnace  equipment,  and  further  shows  that  furnaces  cannot 
well  be  standardized  owing  to  the  great  variety  of  conditions  which 
must  be  met. 

Unit  Furnace  System. — Every  large  factory  employing  heat 
treatment  methods  is  more  or  less  confined  to  a  general  type  of 
product  which  requires  some  particular  and  standardized  treat- 
ment. Such  conditions  would  seem  to  be  naturally  and  readily 
adapted  to  the  small-furnace  unit  practice,  having  elasticity  of  pro- 
duction as  its  general  aim.  Thus  it  is  no  unusual  sight  to  see  tien 
or  fifteen  small  furnaces,  often  on  legs,  of  similar  type  and  size, 
strung  out  in  a  row.  And  yet  if  the  manager  of  that  plant  were  to 


\//////A 


W7777A 


W  J 

1 


K//// 


'///////////// 

4 


FIG.  145. 


visit  a  boiler  room  where  each  of  ten  or  more  boilers  were  each  set 
off  by  itself,  how  caustic  would  be  his  comments.  The  same  prin- 
ciples of  heat  generation,  utilization  and  conservation  are  applica- 
ble to  both.  Not  only  does  the  unit  system  of  furnace  arrangement 
require,  as  a  general  rule,  more  care  and  attention,  involve  more 
steps  for  the  furnace  man,  longer  time  for  heating  and  general 
inefficient  handling,  but  it  also  tends  to  raise  the  cost  of  fuel  out  of 
all  proportion  to  the  product  turned  out.  Radiation  losses  are 
largely  responsible  for  this  last  cost  factor. 

Experience  has  shown  that  three,  or  perhaps  four,  small  single 
furnaces  of  the  same  type  for  the  same  work,  are  about  the  general 
economic  limit  of  the  small-furnace  unit  system. 

To  explain  some  of  the  diagrams  in   Figs.  143,  144  and  145, 


226  STEEL  AND  ITS  HEAT  TREATMENT 

previously  referred  to,  and  to  develop  the  growth  of  the  multiple 
furnace,  we  might  commence  with  a,  b  and  c  of  Fig.  143. 

In  (a)  the  door  opening  is  the  full  width  of  the  chamber,  giving 
opportunity  for  packing  the  charge  to  the  full  width  of  the  open- 
ing; such  a  condition,  as  previously  explained,  is  usually  not  advis- 
able, as  it  tends  to  cut  off  circulation.  It  may  be  remedied  as  in 
(6),  by  placing  jambs  on  each  side  of  the  front. 

In  (c)  there  is  an  opening  at  each  end  of  the  furnace,  allowing 
for  charging  at  one  end  and  discharging  at  the  other,  or  for  work- 
ing the  same  operation  from  both  ends  simultaneously.  Such 
construction  is  thermally  bad  in  that  it  permits  a  direct  draft  from 
one  door  to  the  other,  with  consequent  loss  of  heat.  If  it  were  a 
question  of  doing  the  same  work  from  each  side  at  the  same  time, 
the  layout  might  be  changed  as  in  (d)  or  (e)  to  prevent  the  cold 
air  draft.  Or  if  it  were  a  matter  of  charging  and  discharging  simul- 
taneously without  interference,  the  design  might  better  take  the 
form  of  (f),  having  the  two  openings  at  one  end,  and  widening  the 
furnace  to  make  up  for  loss  in  length.  In  the  latter  construction  the 
door  openings  could  be  smaller,  and  one  side  could  be  worked  out 
while  the  other  side  was  being  charged  and  heated. 

Twin  Chambers. — The  next  logical  step  in  avoiding  the  small  unit 
system  is  to  use  twin  chamber  construction.  For  each  exposed 
wall  of  the  unit  system  which  is  removed,  the  more  can  we  increase 
the  sturdiness  of  construction,  and  there  diminish  the  heat  radia- 
tion loss  of  the  remaining  walls  without  additional  construction 
cost.  Thus  the  development  of  two  furnaces  of  the  (a),  (b),  (c) 
or  (d)  type,  Fig.  143,  will  be  that  as  shown  in  (oa),  (66),  (cc)  and 
(dd).  Such  furnaces  will  have  the  advantages  of  better  heat  applica- 
tion and  conservation,  and  perhaps  of  handling  material,  but  will 
have  the  disadvantage  that  if  one  side  "  goes  down/'  the  other  will 
usually  have  to  go  out  of  commission  also. 

In  some  cases  the  dividing  wall  may  be  omitted  as  in  (g)  and  (h) , 
giving  one  large  hearth.  But  in  this  case  only  one  temperature 
work  can  be  done  at  one  time  in  place  of  the  two  temperatures 
possible  in  the  twin  chamber  construction. 

Furnace  Batteries. — There  are  many  plants  which  uso  small, 
light  furnaces,  built  on  legs  which  might  advantageously  combine 
the  smaller  units  into  batteries  and  obtain  both  better  heat  applica- 
tion and  handling  methods.  Other  things  being  equal,  the  construc- 
tion and  arrangement  of  Fig.  144  (6)  and  (c)  would  be  much  better 
than  that  of  (a). 


HEAT  APPLICATION  227 

Other  designs  as  necessitated  by  definite  conditions  of  material 
handling  or  shop  efficiency  are  illustrated  in  Fig.  145,  and  may  be 
varied,  of  course,  ad  libitum. 

General  Furnace  Considerations.- — Full  data  should  be  gathered 
as  to  the  sizes,  shapes  and  approximate  output,  both  maximum  and 
minimum,  of  the  material  to  be  handled,  besides  the  specific  treat- 
ment desired. 

Careful  consideration  should  then  be  given  as  to  whether  or  not 
the  operations  can  be  made  mechanical,  i.e.,  automatic  or  semi- 
automatic, or  continuous.  From  this  information  the  specific 
dimensions  of  the  furnace  can  be  deduced.  As  a  general  principle, 
it  may  be  stated  that  the  heating  chamber  should  be  large,  but 
with  a  minimum  area  of  exposure  or  door  opening. 

The  furnace  should  be  of  the  best  possible  design  to  suit  the  par- 
ticular work  in  hand  and  under  the  certain  existing  factory  con- 
ditions. The  specific  purpose  for  which  the  furnace  is  to  be  used 
should  be  definitely  decided — whether  for  annealing,  hardening, 
toughening,  carburizing,  etc.,  or  for  a  combination  of  such  opera- 
tions. In  this  connection  it  may  be  said  that  the  maximum  effi- 
ciency of  any  heat  treatment  operation  or  furnace  is  obtained  by 
using  a  furnace  operating  continuously  on  one  class  of  work  at  one 
temperature  and  especially  designed  for  that  purpose. 

Much  thought  should  be  given  to  the  layout  efficiency.  The 
furnaces  should  be  so  arranged  and  such  methods  devised,  that 
the  material  may  be  heated  and  handled  with  the  least  labor  and 
loss  of  time. 

Practical  Notes. — In  concluding  the  discussion  of  heat  applica- 
tion, the  author  would  ask  for  a  thoughtful  consideration  of  the 
following  additional  suggestions  on  heating: 

(1)  The  rate  of  heat  absorption  by  the' object,  under  the  regular 
methods  of  packing  and  furnace  operation,  should  be  obtained.     The 
furnace,  man    should  know  exactly  the  length   of    time  it  takes 
thoroughly  to  saturate  that  piece  of  steel  under  definite  working 
conditions,  and  with  the  furnace  maintained  at  the  desired  tem- 
perature. 

(2)  A  furnace  should  always  be  maintained  at  one  tempera- 
ture, not  permitting  the  indicated  temperature  in  the  furnace  to 
go  higher  than  that  desired  in  the  finished  product.     Forcing  the 
furnace  after  each  discharge  is  bad  practice,  and  tends  to  place  too 
much  confidence  in  the  weak  link — the  human  element. 

(3)  The  pyrometer  and  the  time  clock  should  go  together. 


228  STEEL  AND  ITS  HEAT  TREATMENT 

(4)  Heating  and  handling  methods  should  tend  to  the  prin- 
ciple of  putting  a  cold  piece  in  when  a  hot  piece  is  taken  out;   of 
heating  small  units  at  one  time  (not  meaning  small  furnaces,  but 
units  of  charge)  rather  than  increasing  the  mass;   of  giving  the  last 
piece  in  the  same  amount  of  heat  as  the  first  piece. 

(5)  There  is  no  one  fuel,  furnace,  or  any  other 
will  satisfy  every  condition. 


CHAPTER  X 


CARBON   STEELS 

Foreword. — In  this  and  in  the  following  chapters  on  various 
steels  it  is  the  author's  intention  to  give  the  physical  results  which 
are  representative  of  the  different  steels  and  their  treatment.  Such 
results  have  been  gathered  from  practical  work  and  experiment, 
and  although  the  results  of  various  treatments  will  vary  according 
to  the  individual  steel  and  the  personal  equation  of  the  operator, 
they  may  be  considered  as  fairly  representative  of  the  steel  and  treat- 
ments given. 

Further,  it  must  be  remembered  that  the  size  of  section  or  mass 
of  the  steel  has  a  very  important  influence  upon  the  physical  test 
results.  The  same  results  will  not  be  obtained  in  a  steel  bar  of 
4  ins.  diameter  as  in  a  bar  of  the  same  steel  with  similar  treatment 
and  of  only  1J  ins.  diameter.  Similarly,  different  results  will  be 
obtained  near  the  outer  surface  of  a  large  forging  in  comparison 
with  a  test  taken  near  the  center. 

As  an  example  of  the  effect  of  the  size  of  piece  upon  the  tensile 
strength,  under  the  same  treatment,  we  may  cite  the  following 
examples : 


Diameter 
of  Bar 
Inches. 

Tensile  Strength. 
Lbs.  per  Sq.  In. 

4 

137,000 

l 

132,000 

H 

127,000 

2 

122,000 

2| 

113,000 

3 

105,000 

3| 

100,000 

Hardness  vs.  Maximum  Strength. — The  following  equations 
connecting  maximum  strength,  Brinell  hardness  number  and  sclero- 
scope  hardness  number  have  been  computed  l  from  several  hundred 

1R.  R.  Abbott,  A.  S.  T.  M.,  Vol.  XV,  Part  II,  1915,  p.  43  et  seq. 

229 


230 


STEEL  AND   ITS  HEAT  TREATMENT 


tests  made  with  carbon  steels  of  different  carbon  content  and  heat 
treated  to  bring  out  all  possible  physical  properties: 

(1)  M  =  0.735-28. 

(2)  M=4A    £-28. 

(3)  £  =  5.6    S+14. 

M  =  maximum  strength  in  units  of  1000  Ibs.  per  sq.  in. 
#  =  the  Brinell  hardness  number. 
S  =  the  scleroscope  hardness  number. 

The    maximum    strength    corresponding    to    different    Brinell 
values  as  determined  by  equation  (1)  for  carbon  steels  is  as  follows: 


Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

ICO 

45,000 

350 

227,000 

150 

81,000 

400 

264,000 

200 

118,000 

450 

300,000 

250 

154,000 

500 

337,000 

300 

191,000 

550 

373,000 

The  maximum  strength  corresponding  to  different  scleroscope 
values  as  determined  by  equation  (2),  and  the  corresponding  Brin- 
ell numbers  as  determined  by  equation  (3),  for  carbon  steels,  are 
as  follows: 


Scleroscope. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

20 

60,000 

126 

30 

104,000 

182 

40 

148,000 

238 

50 

192,000 

294 

60 

236,000 

350 

70 

280,000 

406 

80 

324,000 

462 

90 

368,000 

518 

100 

412,000 

574 

VERY  LOW   CAKBON   STEELS:     UNDER  0.15   CARBON 

The  "  dead  soft  "  steels,  about  0.10  per  cent,  carbon,  find  but 
little  application  to  heat-treatment  purposes.  Contrary  to  general 
opinion,  these  steels  do  respond  to  heat  treatment,  although,  of 
course,  to  a  very  limited  extent.  The  best  treatment  to  which  these 
steels  can  be  subjected  is  a  quenching  from  about  1550°  to  1600°  F., 


CARBON   STEELS 


231 


with  or  without  reheating,  the  reheating  being  omitted  when  con- 
ditions (of  strain  caused  by  quenching)  will  permit.  Such  a 
treatment  will  refine  the  grain  and  remove  any  strains  set  up  by 
previous  working;  will  confer  added  toughness;  and  will  put  the 
steel  in  the  best  condition  for  machining.  This  last  is  an  important 
point,  for  this  very  low  carbon  steel,  without  high  manganese  and 
phosphorus,  and  in  either  the  annealed  or  toughened  condition,  often 
does  not  machine  freely,  but  is  apt  to  tear  badly  in  threading  and 
turning  operations.  The  heat  treatment  of  very  low  carbon  steel 
as  applied  to  the  wire  industry  is  discussed  in  a  subsequent  chapter. 

Annealed. — For  annealing  0.1  per  cent,  steel,  heat  as  rapidly  as 
consistent  with  the  size  and  shape  of  the  piece  to  a  temperature 
slightly  above  the  upper  critical  range  of  the  steel,  approximately 
1600°  F.  In  these  very  low  carbon  steels  the  change  into  austenite 
takes  place  very  rapidly,  so  that  it  is  only  necessary  to  allow  the  heat 
to  penetrate  the  steel  at  the  temperature  noted  above.  As  the 
grain  begins  to  coarsen  rapidly  with  increase  in  temperature  and 
length  of  time  held  there,  care  should  be  taken  in  not  overheating 
nor  maintaining  the  annealing  temperature  for  too  long  a  time. 
Cooling  may  be  carried  out  comparatively  rapidly  without  danger 
of  hardening  the  steel.  The  annealing  of  these  very  low  carbon 
steels  is  usually  for  the  purpose  of  relieving  such  strains  as  may  be 
incurred  by  cold  crystallization  or  by  previous  heating  at  a  low- 
red  heat  for  any  great  length  of  time. 

Heat  Treated. — The  results  obtained  from  the  treatment  of  test 
bars  |  ins.  in  thickness  of  acid  open-hearth  steel  of  the  composition: 

Per  Cent. 

Carbon 0.10 

Manganese 0.32 

Phosphorus 0.028 

Sulphur 0.024 

Silicon 0.019 

are  given  in  the  following  table: 


Steel. 

Tensile 
Strength. 
Lbs.  per 
Sq.  In; 

Elastic 
Limit  . 
Lbs.  per 
Sq.  In. 

Elongation. 
Per  Cent, 
in  8  Ins. 

Reduction 
of  Area. 
Per  Cent. 

As  rolled 

51  625 

35500 

34  5 

65  3 

Annealed  

48,800 

31,770 

37.5 

67.5 

Water  quenched  from  1575°  F.  . 

64,720 

45,550 

22.8 

61.15 

1575°  F.  Water/13000  F  

53,055 

36,500 

35.35 

66.05 

232 


STEEL  AND  ITS  HEAT  TREATMENT 


GENERAL  SPECIFICATION,  ANNEALED 


Tensile  Strength. 
Lbs.  per  Sq.  In. 

Elastic  Limit. 
Lbs.  per  Sq.  In. 

Elongation. 
Per  Cent,  in  2  Ins. 

Reduction, 
of  Area. 
Per  Cent. 

45,000 
to 

28,000 
to 

40 
to 

65 
to 

55,000 

36,000 

30 

55 

0.15-0.25   CARBON  STEEL 

This  grade  of  straight  carbon  steel  is  generally  known  to  the  trade 
as  "  machinery  steel,"  and  as  such  has  innumerable  uses  where 
strength  is  not  an  all-important  factor.  The  steel  forges  and  ma- 
chines well.  The  lower  carbons  find  their  greatest  application  in 
the  case-hardening  processes  which  have  been  previously  described. 
The  higher  carbons  are  used  considerably  in  certain  engine  forg- 
ings  such  as  tie  rods,  valve  stems,  nuts,  flanges,  pins,  levers,  etc.; 
for  machine  work  of  various  description;  for  structural  purposes 
in  automobile  construction,  etc. 

Heat  Treated. — Heat  treatment  of  the  lower  carbons  of  this 
range  confers  but  little  additional  strength  except  in  thin  sections, 
but  does  have  a  most  desirable  influence  in  the  refinement  of  grain 
after  forging  or  other  elaboration.  Hardening  should  be  done 
from  a  temperature  exceeding  the  upper  critical  range — which  is 
about  1550°  F.  for  0.15  per  cent,  carbon,  and  about  1525°  F.  for 
0.20  per  cent,  carbon — in  order  to  effect  the  full  absorption  and 
diffusion  of  the  excess  ferrite.  Some  engineers  recommend  quench- 
ing at  1650°  F.  or  even  higher,  but  the  author  believes  that  such 
high  temperatures  are  not  only  detrimental  on  account  of  a  greater 
tendency  to  warping,  oxidation  and  higher  cost  of  treatment,  but 
are  also  unnecessary  metallurgically.  In  other  words,  those  tem- 
peratures should  be  used  which  will  produce  the  most  efficient  com- 
bination of  physical  properties,  refinement  of  grain  and  low  cost  of 
production.  From  the  results  of  extensive  research  work  upon 
0.18  to  0.28  carbon  stock  used  for  automobile  purposes,  and  from  a 
study  of  its  working  out  in  practice,  the  author  recommends  a 
quenching  temperature  of  about  1500°  to  1525°  F.  for  these  steels. 
Temperatures  lower  than  1500°  do  not  bring  out  the  full  effect 
of  the  treatment,  as  is  shown  by  the  following  average  results  (from 
a  large  number  of  tests)  upon  the  same  steel; 


CARBON  STEELS 


233 


Quenched    in    Oil 
from  —  °  F.;  Re- 
heated to  800°  F. 

Tensile  Strength. 
Lbs.  per  Sq.  In. 

Elastic  Limit. 
Lbs.  per  Sq.  In. 

Elongation. 
Per  Cent  in 
2  Ins. 

Reduction 
of  Area. 
Per  Cent. 

1450 
1500 

70,220 
79,590 

43,460 
52,500 

24.1 
25.6 

48.4 
52.6 

With  hardening  temperatures  higher  than  1550°  F.  there  is  prac- 
tically no  increase  in  the  physical  properties  worthy  of  mention, 
and,  moreover,  the  structure  then  begins  to  coarsen  rapidly.  The 
microscope  x  shows  little  or  none  of  the  original  structure  when  the 
steel  has  been  quenched  from  about  1500°  to  1525°  F. 

With  carbons  greater  than  0.18  or  0.20  per  cent.,  and  particu- 
larly if  the  section  is  small,  or  the  manganese  content  is  more  than 
0.60  per  cent.,  the  necessity  of  reheating  or  toughening  after  quench- 
ing becomes  apparent.  Hardening  small  sections,  such  as  are  used 
in  automobile  construction,  from  about  1525°  F.  without  subse- 
quent drawing — especially  if  water  has  been  used  as  the  cooling 
medium — will  produce  an  inherently  brittle  steel.  The  physical 
characteristics  under  these  conditions  will  be  approximately  as 
follows : 

Tensile  strength,  Ibs.  per  sq.  in 90,000  to  110,000 

Elastic  limit,  Ibs.  per  sq.  in 60,000  to    75,000 

Elongation,  per  cent,  in  2  ins 17  to  12 

Reduction  of  area,  per  cent 30  to  15 

By  reheating  to  800°  or  900°  F.  a  considerable  increase  in  tougn- 
ness  and  ductility  is  obtained,  approximating: 


Tensile  strength,  Ibs.  per  sq.  in. .  .  . 

Elastic  limit,  Ibs.  per  sq.  in . 

Elongation,  per  cent,  in  2  ins 

Reduction  of  area,  per  cent 


70,000  to  85,000 
45,000  to  60,000 
35  to  20 
65  to  45 


Cold-rolled  material,  subsequently  given  the  same  heat  treatment 
as  hot-rolled  material  of  the  same  chemical  composition,  will 
usually  show  about  8000  to  10,000  Ibs.  per  square  inch  higher  in 
elastic  limit  and  tensile  strength. 

Characteristic  results  from  commercial  work  are  given  in  the 
following  table: 

1  See  also  page  44. 


234 


STEEL  AND   ITS  HEAT  TREATMENT 


Material. 

Carbon. 

be 
c 
3 
$ 

Quenched 
in  Oil  from 
°F. 

Re- 
heated 
to  °F. 

Tensile 
Strength. 
Lbs.  per 
Sq.  In. 

Elastic 
Limit. 
Lbs.  per 
Sq.  In. 

Elon- 
gation. 
Per 
Cent 
in  2  In. 

Reduc- 
tion of 
Area. 
Per 
Cent. 

General  char-  ..>  .  .  . 
acteristics 

0.18 
to 
0.25 

0.40 
to 
0.80 

1500 
to 
1550 

800 
to 
900 

70,000 
to 
85,000 

45,000 
to 
60,000 

35 
to 

25 

65 
to 
45 

Auto,  lever  

0.18 

0.40 

1650 

800 

70,030 

45,400 

32 

64 

Pressed  auto,  frame. 

0.22 

0.40 

1530 

800 

71,950 

43,400 

29 

56 

Engine  forging  

0  26 

0.28 

1650 

1025 

77,210 

52,200 

28 

65 

Old  rolled  Y±  in.  plate 

0.24 

0.60 

1525 

900 

93,300 

65,250 

20.5 

51 

The  above  remarks  apply  mainly  to  the  smaller  sections  up 
to  2  ins.  in  thickness,  but  are  nevertheless  applicable  in  part  to 
heavy  work.  With  the  increase  in  sectional  area,  the  effect  of  hard- 
ening decreases,  and  for  particularly  heavy  work  may  result  only 
in  a  refinement  of  grain.  Thus,  for  heavy,  oil-treated  forgings, 
toughening  may  not  be  considered  a  necessity;  such  reheating  will, 
however,  relieve  the  strains  which  are  always  inherent  to  quenched 
steels.  Large  forgings  thus  treated  will  show  an  elastic  limit  of 
30,000  to  50,000  Ibs.  per  square  inch,  with  an  elongation  of  35  to 
25  per  cent,  in  2  ins. 

Annealed. — There  is  probably  more  disagreement  and  argument 
as  to  the  proper  annealing  temperatures  for  this  range  of  carbon 
steel  than  for  any  other.  Opinion  and  practice  are  divided  over  the 
use  of  a  comparatively  high  temperature — 50°  to  100°  over  the 
upper  critical  range — or  a  lower  temperature  laying  somewhere 
between  the  Acl  and  Ac3  ranges.  In  this  group  the  Acl  and  Ac3 
ranges  are  widely  separated  and  the  influence  of  the  carbon-mangan- 
ese content  is  rapidly  increasing.  The  high  annealing  temperature, 
1550°  to  1600°  F.  or  more,  will  give  ample  opportunity  for  the 
absorption  of  the  excess  ferrite,  for  diffusion  and  for  equalization. 
On  the  other  hand,  there  is  according  to  some  authorities  a  marked 
increase  in  grain  size  from  1350°  or  1375°  F.  and  upwards. 

The  whole  question  really  depends  upon  the  condition  of  the  steel 
before  annealing.  If  the  "  breaking-down  "  during  elaboration — 
either  rolling  or  forging — has  been  severe,  if  high  temperatures  have 
been  used,  and  if  the  finishing  temperature  has  not  been  just  right, 
a  high  annealing  temperature  may  be  necessary  to  entirely  relieve 
the  strains  and  equalize  the  steel.  On  the  other  hand,  if  the  steel 


CARBON  STEELS 


235 


has  been  carefully  worked  and  the  micrographic  structure  is  fairly 
good,  the  lower  temperatures  will  probably  be  entirely  satisfactory. 
Much  must  be  left  to  the  operator  and  his  own  particular  problem. 
The  main  point  to  bear  in  mind  is  that  the  lowest  temperature 
should  be  used  which  will  produce  the  desired  results. 

If  we  assume  as  average  figures  for  annealed  steel  of  this  cer- 
bon  range: 

Tensile  strength,  Ibs.  per  sq.  in 58,000  to  65,000 

Elastic  limit,  Ibs.  per  sq.  in 28,000  to  35,000 

Elongation  in  2  ins.,  per  cent over  30 

and  compare  these  with  the  results  of  a  tensile  test  taken  from 
the  steel  to  be  annealed,  a  very  good  idea  of  the  degree  and  length 
of  heating  may  be  obtained.  For  example,  the  following  results 
from  If -in.  rounds  for  gun  barrels  show  that  a  high  annealing  tem- 
perature was  not  necessary  in  this  case,  inasmuch  as  the  original 
steel  was  in  excellent  condition. 

Gun  barrel  steel,  If -in.  rounds. 
Carbon,  0.18  per  cent. 
Manganese,  0.50  per  cent. 
Phosphorus,  0.070  per  cent. 
Sulphur,  0.055  per  cent. 
Silicon,  0.055  per  cent. 


Treatment. 

Tensile 
Strength. 
Lbs.  per  Sq.  In. 

Elastic 
Limit. 
Lbs.  per  Sq.  In. 

Elongation. 
Per  Cent. 
In  3  Ins. 

Reduction 
of  Area. 
Per  Cent. 

As  Rolled 

66,750 

33,820 

33.3 

57.6 

Annealed  at 

degrees  F.      for  minutes 

1360-1400 

30 

64,960 

34,050 

38.0 

61.0 

1500 

20 

65,180 

32,930 

38.3 

58.3 

1500 

105 

64,060 

33,150 

39.1 

62.3 

1830 

15 

62,940 

31,810 

35.7 

56.3 

2120 

5 

61,150 

31,580 

33.8 

53.1 

On  the  other  hand,  the  following  cold-rolled  automobile-frame  steel 
was  particularly  "  hard  "  before  annealing  and  required  a  tempera- 
ture of  1550°  F.  to  relieve  thoroughly  the  effect  of  the  cold  work: 

Carbon,  0.24  per  cent. 

Manganese,  0.38  per  cent. 

Phosphorus.  0.028  per  cent.  • 

Sulphur,  0.038  per  cent. 


236 


STEEL  AND   ITS   HEAT    TREATMENT 


Tensile 
Strength. 
Lbs.  per  Sq.  In. 

Elastic 
Limit. 
Lbs.  per  Sq.  In. 

Elongation. 
Per  Cent, 
in  2  Ins. 

Before  annealing 

100  400 

68,500 

18.6 

After  annealing  at  1550°  F  

66,000 

38,100 

37.0 

For  the  average  run  of  annealing  work  for  this  range  of  carbon, 
a  temperature  of  about  1500°  F.  will  be  found  to  give  satisfactory 
results;  individual  cases  must  be  treated  as  such. 

0.25-0.35  CARBON    STEEL 

Steel  containing  from  0.25  to  0.35  per  cent,  carbon  is  known 
as  soft-forging  steel  and  is  used  principally  for  structural  ptirposes 
in  infinite  variety.  It  responds  in  a  most  satisfactory  manner  to 
welding,  forging  and  machining,  and  may  be  vastly  improved  by 
proper  heat  treatment.  Under  skillful  treatment,  the  variety  of 
combinations  of  -strength  and  ductility  are  to  be  had  in  probably 
no  other  range  of  carbons. 

Relative  to  static  strength,  some  really  wonderful  results— for 
straight  carbon  steels — in  the  way  of  high  tensile  strength  with  high 
ductility  have  been  obtained  from  heat-treated  (oil  quenched  and 
toughened)  forgings  of  0.30  to  0.35  per  cent,  carbon.  The  follow- 
ing results,  obtained  from  the  center  of  a  5-in.  electric  car,  heat- 
treated  axle,  the  #xle  being  selected  at  random  from  a  group  of 
about  one  hundred  forgings,  give  an  idea  of  the  extent  to  which 
proper  heat  treatment  may  develop  the  physical  properties; 

Electric  Car  Axle,  0.32  Carbon,  Acid  Steel 

Tensile  strength,  Ibs.  per  sq.  in 91,700 

Elastic  limit,  Ibs.  per  sq.  in 61,620 

Elongation,  per  cent,  in  2  ins 33.5 

Reduction  of  area,  per  cent 48.1 

In  the  hardened  condition — without  subsequent  tempering — 
these  steels  may  be  used  for  gears.  In  the  toughened  condition 
these  steels  present  the  maximum  resistance  to  fatigue  and  other 
dynamic  stresses,  as  represented  by  alternating  impact  and  other 
tests,  over  any  of  the  straight  carbon  steels;  the  dynamic  strength 
probably  apexes  at  about  0.30  per  cent,  carbon,  as  far  as  the  author 
can  judge  from  his  own  researches  and  from  the  work  and  conclusions 
of  others. 


CARBON   STEELS  237 

Untreated. — In  the  untreated  condition,  with  standard  man- 
ganese, phosphorus  and  sulphur,  the  average  tensile  strength  of 
these  steels  will  be  about  as  follows: 

Carbon.  Acid  Steel.  Basic  Steel. 

0.25  to  0.30  67,000  to  78,000  63,000  to  72,000 

0.30  to  0.35  69,000  to  83,000  65,000  to  74,000 

Rolled  plates,  from  2  to  4  ins.  thick,  made  of  basic  steel  with  0.25 
to  0.35  per  cent,  carbon  and  about  0.40  per  cent,  manganese,  will 
usually  fulfill  the  following  specifications : 

[Tensile  strength,  Ibs.  per  sq.  in 65,000  to  75,000 

Elastic  limit,  Ibs.  per  sq.  in 33,000  to  37,000 

Elongation,  per  cent,  in  2  ins 30  to  25 

Reduction  of  area,  per  cent 50  to  36 

These  results  may  also  be  considered  as  generally  applicable  to 
untreated  steel  of  this  analysis,  but  which  has  had  more  or  less 
elaboration  or  working. 

Heat  Treated. — The  upper  critical  range  decreases  from  about 
1500°  F.  for  0.25  per  cent,  carbon,  to  about  1425°  F.  for  the  0.35 
per  cent,  carbon  steel.  Practical  experience  has  shown  that  a 
quenching  temperature  of  1500°  to  1525°  F.  for  the  lower  carbons  of 
this  range,  and  1450°  to  1500°  F.  for  the  higher  carbons  will  give 
satisfactory  results  under  ordinary  conditions.  If  the  heating  has 
been  conducted  uniformly  and  not  too  rapidly — especially  when 
approaching  the  maximum  temperature — the  original  structure  of 
the  steel  should  be  entirely  eliminated,  as  the  temperatures  recom- 
mended are  distinctly  above  the  upper  critical  range.  Never- 
theless, some  metallurgists  prefer  to  quench  these  steels  from  a 
higher  temperature,  say  1575°  to  1600°  F.,  in  order  to  make  cer- 
tain of  the  complete  change  in  structure  and  to  obtain  a  maximum 
hardening  effect.  In  either  case,  intelligent  furnace  operation  and 
heat  control  will  probably  be  the  governing  factor  rather  than  the 
indicated  furnace  temperature  or  mere  theorizing. 

lor  forgings  in  which  especially  high  qualities  are  desired,  double 
quenching  will  produce  a  refinement  of  grain  and  correspondingly 
higher  elastic  limit  and  ductility  than  are  usually  obtained  by  the 
single  treatment.  The  temperatures  recommended  for  this  range 
of  carbons  are : 

1.  Jirst  quenching  'from  1600°  F.,  or  from  1500°  to  1550°  F. 
if  the  higher  quenching  should  prove  too  drastic. 


238  STEEL  AND   ITS  HEAT  TREATMENT 

2.  Second  quenching  from  1425°  to  1450°  F.,  followed  by 

3.  Suitable  toughening  according  to  the  size  of  piece  and 

physical  properties  desired. 

The  results  to  be  obtained  from  heat  treatment  will  vary  largely 
for  this  range  of  carbon  in  particular,  due  to  such  influence  as  the 
increase  of  a  few  points  in  the  carbon  content  (particularly  noticeable 
in  these  mild  steels),  the  size  of  the  section,  the  quenching  medium, 
and  so  forth.  The  results  given  under  the  0.15  to  0.25  carbon  range, 
and  under  the  0.35  to  0.45  carbon  range  to  follow,  may  be  used  as  a 
general  measure  of  the  carbons  under  discussion.  Stated  roughly, 
these  carbons  will  give  elastic  limits  ranging  from  35,000  to  80,000 


FIG.  146. — 0.28  per  cent.  Carbon  Steel.     X39.     (Campbell.) 

Ibs.  per  square  inch,  with  corresponding  elongations  of  30  to  10  per 
cent,  in  2  ins. 

Annealed. — As  has  been  previously  explained,  heating  for  anneal- 
ing to  just  above  the  Acl  (lower)  critical  range  will  refine  the  ground- 
mass  only,  while  complete  refinement  is  shown  by  the  disappearance 
of  the  ferrite  and  network  beyond  the  upper  critical  range  (Ac3). 
As  an  example  of  this,  examine  the  photomicrographs  of  a  basic 
open-hearth  steel  containing  0.28  per  cent,  carbon  and  0.52  per  cent, 
manganese,  as  shown  in  Figs.  146,  147  and  148.  The  first  photo- 
graph shows  the  original  steel  with  its  coarse,  weak  structure.  Fig. 
147  shows  the  same  steel  annealed  at  1425°  F.,  or  just  over  the  Acl 
range;  the  pearlitic  ground-mass  has  been  entirely  refined,  but  there 
still  remains  the  unabsorbed  and  undiffused  excess  ferrite.  Fig. 


CARBON   STEELS 


239 


148  shows  the  same  still  heated  to  1520°  F.  and  slow  cooled  in  the 
same  manner;  but  in  this  case  the  structure  has  been  entirely  changed 
and  refined  by  heating  to  a  temperature  over  the  upper  critical  range. 


.         . 


FIG.  147.— 0.28  per  cent.  Carbon  Steel  Annealed  at  1425°  F. 
X39.     (Campbell.) 


FIG.  148. — 0.28  per  cent.  Carbon  Steel  Annealed  at  1520°  F. 
X39.     (Campbell.) 

Practical  experience  has  shown  that  a  temperature  of  1500°  to 
1525°  F.  will  give  excellent  results  for  the  full  annealing  of  steels 
within  this  range  of  carbons.  On  account  of  the  hardening  effect 
of  air  cooling  steeis  with  over  0.20  per  cent,  carbon  when  in  small 


240  STEEL  AND   ITS  HEAT  TREATMENT 

sections,  these  steels  should  be  slow  cooled,  either  in  the  furnace, 
in  lime  or  in  ashes. 

In  regard  to  the  physical  properties  to  be  obtained  from  the 
annealing  of  these  steels,  the  lower  carbons  of  this  range  should 
always  meet  tthe  U.  S.  Government  specification  of: 

Tensile  strength,  Ibs.  per  sq.  in 60,000 

Elastic  limit,  Ibs.  per  sq.  in 30,000 

Elongation,  per  cent,  in  2  ins 30 

while  the  higher  carbons  will  usually  give : 

Elastic  limit,  Ibs.  per  sq.  in 35,000  to  45,000 

Elongation,  per  cent,  in  2  ins 22  to  32 

Reduction  of  area,  per  cent 30  to  60 

0.35-0.45  CARBON  STEEL 

Straight  carbon  steels  with  0.35  to  0.45  per  cent,  carbon  are 
particularly  suited  to  medium  and  heavy  forgings  for  which  the 
lower  carbons  would  not  give  sufficient  strength,  and  for  which  it 
is  also  not  desirable  to  use  water  quenching  on  account  of  the  possi- 
bility of  starting  incipient  cracks  or  strains.  This  steel  is  commonly 
used  for  high-duty  and  moving  machine  parts;  for  axles,  side  bars, 
crankpins  and  other  locomotive  forgings ;  for  guns  and  gun  forgings ; 
for  crank  shafts,  driving  shafts  and  similar  automobile  parts;  and 
for  general  structural  purposes  requiring  the  combination  of  maxi- 
mum strength  with  minimum  brittleness.  It  has  excellent  dynamic 
strength,  although  probably  not  quite  so  much  as  the  previous 
class  of  0.25-0.35  carbon.  Steel  with  0.40  carbon  according  to 
Robin  x  presents  the  greatest  resistance  to  abrasive  action  (wear) . 
These  steels  are  easy  to  machine  when  in  the  annealed  or  soft- 
toughened  condition,  but  should  not  be  used  for  screw  machine  stock. 

The  upper  critical  range  temperature  of  this  steel  is  about  1425°  F. 
to  1400°  F. 

Untreated. — The  average  untreated  American  open-hearth  steel 
with  standard  manganese,  phosphorus  and  sulphur  will  average 
about  as  follows  in  tensile  strength: 

Carbon.  Acid  Steel.  Basic  Steel. 

0.35  to  0.40  78,000  to    92,000  70,000  to  78,000 

0.40  to  0.45  87,000  to  100,000          76,000  to  89,000 

1 J.  Robin,  Inst.  Journ.,  II,  1910. 


CARBON  STEELS 


241 


Annealed. 


Remarks. 

C. 

Mn. 

Phos. 

Sul. 

Tensile 
Strength. 
Lbs.  per 
Sq.  In. 

Elastic 
Lir  it. 
Lbs.  per 
Sq.  In. 

Elong- 
ation. 

%in 
2  Ins. 

Red.  of 
Area. 
Per 

Cent. 

General  limits.  .  .  . 

0.35 
to 
0.45 

not 
over 
0.70 

under 
0.045 

under 
0.045 

70,000 
to 
85,000 

38,000 
to 
50,000 

28 
to 
20 

55 

to 
40 

Forged  gun  jacket 
acid  steel  

0.35 

0.25 

0.038 

0.019 

77,080 

39,500 

27 

Forged  gun  jacket 
basic  steel  

0.43 

0.22 

tr. 

0.023 

78,180 

43,100 

25.5 

8-in.      axle      acid 
steel     annealed 
at  1400°  F  

0.42 

0.51 

78,420 

47,460 

28 

54.5 

Heat-treated. — Large  sections,  when  quenched  in  good  mineral  oil 
from  1400°  to  1500°  F.,  and  toughened  at  900°  to  1200°  F.  (according 
to  the  carbon  content  and  largest  section),  should  always  meet  the 
specification  of  85,000—50,000—22—45.  The  following  tests  taken 
from  large  forgings  show  the  variety  of  combinations  of  strength 
and  ductility  which  may  be  obtained: 


Forging. 

Carbon. 

Treatment. 

Tensile 
Strength. 
Lbs.  per 
Sq.  In. 

Elastic 
Limit. 
Lbs.  per 
Sq.  In. 

Elonga- 
tino. 
Per  Cent, 
in  2  Ins. 

Red.  of 
Area. 
Per  Cent. 

Gun  jacket 
Axle  

0.35 
0.41 

1500-0/1200 
1450-w/lOOO 

109,560 
90,250 

65,090 
54,575 

16.5 
25  4 

52  4 

Gun  jacket 
Shaft  

0.43 
0.42 

1500-0/1200 
1525-  0/1300 

111,100 
82,040 

69,700 
57,060 

17.0 
29.0 

55.0 

o  =  oil.     w  =  water. 

It  is  always  advisable  to  keep  the  drawing  temperature  as  near 
1200°  to  1250°  F.  as  possible,  not  only  because  it  is  easier  for  the 
furnace  operator  to  obtain  more  accurate  temperature  control  at 
these  more  readily  distinguished  "  reds,"  but  also  on  account  of  the 
greater  dynamic  strength  which  is  obtained  by  the  use  of  the  higher 
drawing  temperatures. 

The  results  obtained  from  the  water  quenching  from  1450°  F. 
and  subsequent  toughening  of  small  rounds  of  0.40  per  cent,  carbon 
steel  are  given  in  the  chart  in  Fig.  149. 


242 


STEEL  AND   ITS  HEAT   TREATMENT 


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Chemical  Analysis:  Critical  Ranges:  Size  of  Section  Treatment: 
C.  0.40  Acl  1330°  1  inch  round  Quenched  in 
Mn.  0.60  Ac2  water  from 
P.  0.02  Ac3  1410°  1450,°  and 
S.  0.03  drawn  as 
-.  given. 

FIG.  149.  —  Normal  Characteristics  of  0.40  Carbon  Steel,  Heat  Treated. 

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CARBON  STEELS  243 

LOCOMOTIVE   AXLES 

Locomotive  axles  and  other  heavy  forgings  used  in  locomotive 
construction  are  illustrative  of  the  treatment  of  pieces  of  large 
section  of  the  above  carbon  range. 

Heat-treated  Axles. — For  heat-treated  axles  the  carbon  content 
will  range  between  0.35  and  0.50  per  cent.,  and  with  such  steel 
the  treatment  may  generally  be  adjusted  to  meet  the  standard 
specification  of: 

Tensile  strength 85,000  Ibs.  per  sq.  in. 

Elastic  limit 50,000  Ibs.  per  sq.  in. 

Elongation 22  per  cent,  in  2  ins. 

Reduction  of  area. .  . .  • 45  per  cent. 

The  quenching  of  the  axles,  usually  from  temperatures  of  1400° 
to  1500°  F.,  is  mainly  a  proposition  of  correct  heat  application  and 
efficient  handling.  Both  oil  and  water  are  used  extensively  for 
hardening  axles.  Water  will  bring  out  the  full  effect  of  heat  treat- 
ment by  giving  the  highest  tensile  test  properties  of  which  the  steel 
is  capable;  with  the  same  ductility,  oil  quenching  will  give  lower 
tensile  values  than  water  quenching.  A  steel  with  lower  carbon 
content  may  more  logically  be  used  with  water  quenching  than  with 
oil.  Water  requires  no  expensive  cooling  nor  circulation  system, 
and  has  practically  no  cost  of  upkeep  or  replenishment,  as  all 
these  may  be  regulated  by  the  intake  of  fresh,  cold  water.  On 
the  other  hand,  many  engineers  severely  condemn  the  use  of  water 
in  that  it  is  too  harsh  in  its  action  upon  large  masses  of  steel — as  in 
axles,  that  cracks  are  more  liable  to  develop,  and  that  internal 
strains  are  set  up  which  often  are  not  always  entirely  relieved  by 
the  reheating  or  toughening.  Oil  is  much  the  safer  quenching 
medium  to  use  for  axles,  will  give  more  uniform  results,  and  should 
only  be  replaced  by  water  for  economic  reasons. 

The  hardened  axles  are  charged  while  still  warm  into  the  re- 
heating furnace,  which  is  maintained  at  such  temperature  as  will 
relieve  all  strains  set  up  in  the  hardening  and  at  the  same  time 
give  the  physical  properties  desired.  This  temperature  will  vary 
between  900°  and  1200°  F.,  depending  upon  the  chemical  composi- 
tion of  the  steel.  The  higher  the  drawing  temperature,  the  more 
ductile  the  steel  and  apparent  coarseness  of  the  grain,  due  to  the 
transformation  of  the  transition  constituents  troostite  and  sor- 
bite  into  pearlite  plus  free  ferrite.  Straight  carbon  steels  quenched 
in  water  and  drawn  at  1000°  or  1100°  F.  will  be  entirely  sorbitic,  but 


244  STEEL  AND  ITS  HEAT  TREATMENT 

at  1200°  may  show  considerable  free  ferrite.  Too  much  emphasis 
cannot  be  given  to  the  necessity  of  keeping  a  uniform  temperature 
and  allowing  sufficient  time  for  the  heat  thoroughly  to  penetrate  the 
axle.  It  is  then  preferable  for  the  axles  to  cool  with  the  furnace, 
rather  than  to  remove  them  while  still  at  the  toughening  temperature. 
The  temperatures  used  by  one  manufacturer  of  acid  steel  axles 
and  other  large  forgings  to  meet  the  standard  A.  S.  T.  M.  specifica- 
tions are  as  follows:  after  quenching  in  oil  from  1450°  F.,  reheat  as 
below  and  air-cool  or  cool  in  ashes. 

0.42  to  0.45  per  cent,  carbon 1175°  F. 

0.38  to  0.42    "     "  " 1125°  F. 

0.33  to  0.38    "     "  "      1075°  F. 

0.28  to  0.33    "     "  "      1000°  F. 

The  following    are    characteristic    tests  from  Open-hearth  steel 
locomotive  axles  (Penna.  R.R.) : 

0.41  Carbon 

•t?nrtrfM*  Treated, 

orged .    j  500o  water/  !  000°  F. 

Tensile  strength 73,627  90,250 

Elastic  limit 31,505  54,575 

Elongation 31.6  25.4 

Reduction  of  area. .  43.6  52.4 


0.50  Carbon 


Treated, 
water/12000  F. 


Tensile  strength  ..............  83,430  90,092 

Elastic  limit  .................  34,370  53,655 

Elongation  ...................       22.6  27.1 

Reduction  of  area  .............       30.0  52.8 

"  Tempered  "  Axles.  —  For  want  of  a  better  name,  under  this 
heading  might  be  included  such  as  are  treated  by  the  "  Coffin  "  or 
similar  processes.  The  main  principle  consists  in  heating  the  axle  as 
usual  for  hardening,  then  immersing  in  the  quenching  bath  for  a  cal- 
culated number  of  seconds,  immediately  withdrawing,  and  allowing 
the  heat  from  the  interior  of  the  axle  to  "  temper  "  the  part  which 
had  been  hardened  by  the  short  immersion  in  the  oil  or  water. 
This  process  has  been  developed  to  such  a  nicety  that  surprisingly 
uniform  results  may  be  obtained  if  the  test  is  always  taken  from  the 
same  relative  place,  such  as  half-way  between  the  center  and  the 
outside,  The  main  arguments  for  the  process  are  that  it  is  simple 


CARBON  STEELS  245 

and  that  the  axle  will  have  a  tough  (annealed)  core,  and,  at  the  same 
time,  a  hard  wearing  surface.  Non-uniformity  of  structure  is  the 
principal  argument  of  those  condemning  the  process,  and  on  account 
of  the  inevitable  "  human  equation  "  which  enters  into  it,  would 
seem  to  be  not  without  justice  in  many  instances. 

Annealed  Axles. — The  axles  are  heated  up  slowly  and  uniformly 
to  a  temperature  slightly  in  excess  of  the  upper  critical  range,  main- 
tained at  this  temperature  for  sufficient  time  for  the  steel  to  respond 
to  the  heat,  and  then  cooled  with  the  furnace.  If  the  working 
and  the  finishing  temperature  during  forging  have  been  adjusted 
so  as  to  give  a  fine  grain  to  the  steel,  besides  good  physical  test  re- 
sults, it  will  be  found  that  heating  to  a  temperature  over  the  critical 
range  may  not  be  necessary  in  many  instances.  Where  it  is  neces- 
sary to  anneal  large  numbers  of  heavy  axles,  the  hot  axles  may  be 
removed  quickly  to  a  pit  and  covered  with  lime  or  ashes.  Annealing 
alone  will  generally  overcome  the  strains  set  up  in  the  previous  proc- 
esses of  manufacture,  but  it  does  not  bring  out  the  higher  physical 
properties  of  which  the  steel  is  capable.  Annealed  axles  will  show 
pearlite  and  free  ferrite,  the  apparent  size  of  the  ferrite  grains  de- 
pending upon  the  rate  of  cooling  and  the  time  thus  given  for  the 
ferrite  to  separate  out  from  the  matrix. 

In  order  to  obtain  the  required  tensile  strength  upon  anneal- 
ing it  is  necessary  to  use  a  steel  of  higher  carbon  content  than  that 
used  for  full  heat  treatment.  A  tensile  strength  of  80,000  Ibs.  per 
square  inch  will  require  a  0.50  carbon  steel  or  higher.  A  temperature 
of  1500°  F.  is  generally  recommended  (A.  S.  T.  M.)  for  annealing 
0.40  to  0.60  per  cent,  carbon  steel,  but  since  the  critical  range  of 
this  steel  is  about  1400°  F.  or  a  little  under,  an  annealing  temperature 
of  1400°  to  1450°  F.  will  give  a  better  fracture,  together  with  a  better 
combination  of  tensile  strength  and  ductility. 

The  author  has  had  much  better  results  with  the  lower  temper- 
ature, although  the  time  required  for  the  annealing  generally  is  longer. 
The  following  results  from  acid  open-hearth  steel  will  show  the 
effect  of  the  lower  temperature  anneal : 


Heat  4261 

Annealed  at  1400° 

Carbon,  0.42  per  cent  
Manganese,  0.51  per  cent  .  .  . 
Phosphorus,  0.034  per  cent  .  . 
Sulphur  0.028  per  cent  

Tensile  strength  
Elastic  limit  
Elongation  in  2  ins  ... 
Reduction  of  area.  .  .  . 

78,420  Ibs.  per  sq.  in. 
47,460  Ibs.  per  sq.  in. 
28  per  cent. 
54.6  per  cent. 

246  STEEL  AND   ITS  HEAT  TREATMENT 

Failures  of  Heat-treated  Axles. — Aside  from  piping,  segrega- 
tion, and  other  impurities  in  the  steel,  improper  heat  treatment  is 
the  active  cause  of  failure  of  both  heat-treated  carbon  and  alloy 
steel  axles.  Unequal  or  insufficient  heating  in  either  the  hardening 
or  toughening  processes  will  produce  unequal  stresses,  which  in 
turn  will  sooner  or  later  result  in  failures.  These  failures  are 
always  transverse,  and  never  longitudinal.  Water  quenching  large 
sections  has  a  strong  tendency  to  produce  cracks,  often  not  appear- 
ing on  the  surface,  and  which  may  open  up  when  subjected  to  the 
heavy  duty  and  "  pounding  "  when  placed  in  service.  Such  defects 
may  be  sometimes  discovered  by  the  drop  test,  but  its  expense 
prevents  many  railroads  from  using  it  for  the  test  of  every  axle. 
Heat-treated  axles,  when  given  ample  reduction  in  the  forging 
operation,  carefully  and  uniformly  heated  to  the  proper  tempera- 
tures, and  held  at  those  temperatures  for  a  time  sufficient  for  the 
steel  to  respond  throughout,  should  prove  vastly  superior  to  un- 
treated or  annealed  axles. 

On  the  other  hand,  the  engineering  departments  of  many  rail- 
roads have  become  considerably  alarmed  over  the  frequent  failures 
of  so-called  "  heat-treated  "  axles,  and  many  have  absolutely  refused 
to  have  anything  to  do  with  axles  which  have  been  oil  or  water 
quenched.  This  really  serious  phase  of  the  axle  question  has  led 
to  the  investigation  of  the  possibilities  of  hardening  such  forgings 
in  air  or  steam.  Surprisingly  good  results  have  been  obtained  by 
methods  based  upon  this  system — and  from  the  technical  stand- 
point are  indeed  remarkable,  since  a  considerable  toughening  heat  is 
necessary  with  even  0.40  per  cent,  carbon  steel. 

0.45-0.60  CARBON   STEEL 

Treatment  of  Large  Sections. — As  the  carbon  content  is  pro- 
gressively increased  beyond  0.45  per  cent.,  its  effect  becomes  quite 
noticeable  in  the  added  brittleness  of  the  steel.  This  is  strongly 
illustrated  by  the  fact  that  a  general  study  of  heat-treatment  prac- 
tice will  show  that  there  is  very  little  quenching  of  large  sections 
when  the  carbon  content  exceeds  the  0.50  per  cent.  mark.  The 
dangers  to  be  encountered,  both  in  the  treatment  itself  and  by 
possible  fracture  in  service,  almost  prohibit  such  treatment  of  large 
sections.  Any  increase  in  static  strength  which  can  be  obtained  by 
quenching  and  toughening  is  most  certainly  acquired  with  the 
ever-present  danger  of  cracking,  or  of  starting  incipient  cracks. 
For  these  reasons  it  is,  therefore,  apparent  that  the  full  heat  treat- 


CARBON   STEELS 


247 


ment  of  large  sections,  even  though  it  may  bring  out  higher  physical 
characteristics  in  the  steel — as  is  shown  by  the  subsequent  figures 
obtained  from  the  treatment  of  a  0.50  per  cent,  carbon  axle — is 
becoming  less  and  less  of  a  factor  in  steels  of  these  carbons. 

0.50  Per  Cent.  Carbon  Axle 


Forged. 

Quenched  in  Water 
from  1400°  F. 
Toughened  at  1200° 

Tensile  strength,  Ibs.  per  sq.  in  
Elastic  limit   so   in 

83,430 
34370 

90,090 
53655 

Elongation  per  cent,  in  2  ins 

22  6 

27  1 

Reduction  of  area,  per  cent  

30.0 

52.8 

Tempering,  and  Small  Sections. — On  the  other  hand,  the  harden- 
ing and  tempering  (as  distinguished  from  toughening)  of  the  smaller 
sections,  such  as  gears,  dies,  etc.,  begins  to  take  an  important  place 
in  heat-treatment  work  with  these  carbons.  In  such  cases  the 
increased-carbon  content  brings  about  an  inherently  possible  wearing 
hardness  which  is  developed  by  hardening  and  tempering.  The 
medium  and  smaller  size  sections  may  be  satisfactorily  hardened 
in  water  with  but  a  small  proportion  of  the  danger  which  would 
inevitably  result  from  the  water  quenching  (or  even  oil  quenching) 
of  larger  sections.  And,  by  varying  the  reheating  temperatures, 
the  following  approximate  physical  results  may  be  obtained: 


Elastic  limit,  Ibs.  per  sq.  in....  .  . 

Elongation,  per  cent,  in  2  ins.  .. 
Reduction  of  area,  per  cent 


.   50,000  to  110,000 
20  to  5 
50  to  15 


Annealing. — The  commrecial  annealing  of  steel  of  say  0.50  to 
0.60  per  cent,  carbon  will  give  a  variety  of  results  which  in  them- 
selves have  proven  a  stumbling  block  for  many  a  heat  treater. 
This  is  largely  due  to  the  prominent  part  and  effect  of  different  rates 
of  cooling  in  relation  to  the  size  or  mass  of  the  steel.  To  illus- 
trate: 6X6  in.  billets  of  0.50  to  0.55  carbon  which  have  been  heated 
to  1400°  F.  and  furnace  cooled  will,  in  general,  meet  the  specifica- 
tions of 

Tensile  strength 80,000 

Elastic  limit 40,000 

Elongation 22 

Reduction  of  area .  35 


248  STEEL  AND   ITS  HEAT  TREATMENT 

On  the  other  hand,  smaller  sections,  annealed  in  the  same  manner 
and  in  the  same  furnace,  will  according  to  their  size,  give  physical 
results  varying  anywhere  between 

Elastic  limit 45,000  to  60,000 

Elongation 20  to  15 

Reduction  of  i  area 40  to  30 

In  other  words,  the  extreme  variability  in  the  rate  of  cooling, 
as  dependent  upon  the  size  of  section  and  mass  of  the  steel,  its  rela- 
tion to  the  size  of  the  furnace,  the  degree  to  which  the  cooling  of 
the  furnace  may  be  controlled,  and  numerous  other  related  factors 
make  the  commercial  annealing  of  these  steels  an  individual  problem 
as  far  as  actual  physical  results  are  concerned. 

It  is  therefore  always  advisable,  if  specific  physical  results  must 
be  obtained  by  annealing  (used  in  the  broad  interpretation  of  the 
term) ,  to  take  first  a  preliminary  test  of  the  steel  in  the  condition  as 
received.  From  such  results  it  will  then  be  evident  how  much  the 
steel  must  be  "  let  down,"  and  the  proper  reheating  temperature 
may  be  judged  from  previous  experience  or  by  experiment.  Al- 
though annealing  at  a  temperature  under  the  critical  range  will 
not  change  the  general  structure  of  a  pearlitic  steel,  it  will  relieve 
the  strains  and  stresses,  and  thereby  improve  the  steel.  But  fur- 
ther, the  previous  elaboration,  such  as  rolling  or  forging,  which  the 
steel  has  undergone,  will,  in  a  majority  of  cases  in  actual  practice, 
leave  the  steel  in  more  or  less  of  a  sorbitic  state.  Under  such 
conditions,  a  reheating — or  commercial  annealing — will  actually 
change  the  physical  results,  even  though  the  annealing  temperature 
is  under  the  critical  range. 

Such  commercial  annealing  or  reheating  temperatures  may  vary 
from  900°  F.  and  upwards  through  the  upper  critical  range.  In  the 
author's  experience  there  is  little  or  no  change  in  the  physical  test 
results  through  the  annealing  of  such  steel  at  temperatures  under 
900°  F.  or  thereabouts.  But  from  this  temperature  upwards 
the  sorbitic  constituents  will  gradually  coagulate  into  the  pearlite 
and  ferrite,  with  a  corresponding  lowering  of  the  static  strength 
and  increase  in  the  ductility.  Consequently,  by  regulating  the 
commercial  annealing  temperature,  the  physical  results  may  be 
"  let  down  "  to  the  desired  limits. 

If  it  is  desired  to  change  entirely  the  structure  of  the  steel  and 
to  obtain  the  finest  grain  size  possible,  with  maximum  ductility,  it 


CARBON   STEELS  249 

will  be  necessary  to  anneal  the  steel  at  a  temperature  slightly  in 
excess  of  the  upper  critical  range,  followed  by  slow  cooling. 

The  influence  of  the  rate  of  cooling,  as  exerted  by  air  cooling, 
is  manifested  in  the  peculiar  statement  that  the  tensile  strength  of 
these  hard  forging  steels  may  be  actually  raised  by  annealing  (as 
distinguished  from  quenching).  It  is  a  well-known  fact  that  steel 
of  such  carbon  content  when  cooled  in  air  at  a  more  or  less  rapid 
rate  through  the  critical  range  will  take  on  a  noticeable  degree  of 
hardness.  The  author  has  found  that  this  fundamental  principle 
may  be  applied  to  great  advantage  in  the  treatment  of  axles — with, 
of  course,  certain  modifications — and  that  it  is  even  necessary  to 
reheat  or  toughen  in  order  to  lower  the  tensile  strength  and  obtain 
the  proper  ratio  of  static  strength  to  ductility.  Such  a  process  is 
now  being  developed  by  a  large  manufacturer  of  axles,  and  will  in 
all  probability  have  an  influence  upon  the  heat  treatment  of  axles 
and  other  forgings  of  large  section. 

SHRAPNEL 

Shrapnel  are  illustrative  of  this  range  of  carbon  and  of  pieces 
of  medium  section. 

The  current  specifications  for  foreign  shrapnel  cover  a  wide  range 
of  physical  properties,  varying  between  80,000  and  140,000  Ibs. 
per  square  inch  in  tensile  strength,  with  20  to  8  per  cent,  elongation. 
The  chemical  composition  of  the  steel  used  will  be  approximately 
between  0.50  and  0.60  per  cent,  carbon,  although  the  extreme  limits 
are  0.35  to  0.8  per  cent.,  0.4  to  1.0  per  cent,  manganese,  phosphorus, 
sulphur  and  silicon  about  normal,  and  with  or  without  the  addition 
of  chrome  or  nickel.  Thus,  according  to  the  physical  or  chemical 
specifications  worked  under,  some  shrapnel  manufacturers  have  been 
able  to  meet  their  particular  specifications  without  any  treatment 
except  perhaps  cooling  the  cases  in  lime  after  forming;  others  have 
had  to  anneal,  or  harden  and  temper;  while  still  others  have  add 
to  carry  out  all  three  heating  operations. 

There  is  nothing  unusual  in  the  heat  treatment  required.  The 
proposition  in  short  is  merely  one  of  proper  heat  application  in  fur- 
naces of  correct  design  and  construction;  and  yet  one  may  see 
almost  any  and  every  kind  of  a  furnace  being  operated  in  almost 
any  and  every  kind  of  a  way  except  the  right  one,  with  the  result 
of  large  rejections. 

The  latest  and  a  very  efficient  type  of  furnace  for  this  work  is 


250 


STEEL  AND   ITS  HEAT  TREATMENT 


designed  on  underfired  principles,  with  a  continuous  and  auto- 
matic charging  and  discharging  of  the  shrapnel.  The  cycle  for  the 
complete  hardening  and  drawing  is  as  follows:  one  man  places 
the  rough-formed  shells  on  the  charging  platform  of  the  hardening 
furnace,  as  shown  in  Fig.  149a;  an  automatic  device  takes  the 
shrapnel  into  and  through  the  heating  chamber  at  a  specified  and 
predetermined  rate;  the  heated  shrapnel  are  then  discharged  con- 
tinuously from  the  furnace  into  an  oil-quenching  bath,  as  shown  in 


FIG.  149a. — Automatic  Hardening  Furnaces,  Charging  End,  Working  on 
3-in.  Shrapnel. 

Fig.  1496,  from  which  they  are  removed  by  a  conveyor  system  and 
delivered  to  a  table  in  front  of  the  second  or  drawing  furnace.  Here 
another  man  takes  them  from  the  table  and  places  them  on  the 
charging  table,  as  shown  in  Fig.  149c;  the  shrapnel  then  follow 
the  same  course  as  in  the  first  furnace,  except  that  they  are 
received  from  the  furnace  into  wheelbarrows  or  similar  means  of 
taking  them  away,  as  shown  in  Fig.  149d.  The  actual  temperature 
of  the  shrapnel  in  the  case  of  one  plant  was  1500°  F.  for  hardening 
and  850°  F.  for  the  tempering  or  drawing  operation. 


CARBON   STEELS 


251 


FIG.  1496. — Hardening  Furnaces,  Discharge  End;  Oil  Quenching  Tanks  at  the 

Left. 


FIG.  149c. — Tempering  Furnaces — Charging  End — Conveyer  from  Quenching 

Tank. 


252 


STEEL  AND   ITS  HEAT  TREATMENT 


In  connection  with  the  above  equipment  a  comparison  between 
the  old  hand  method  and  the  new  automatic  appliances  is  extremely 
interesting.  The  following  are  actual  tests  made  in  the  same  plant 
on  3.3-in.  shrapnel: 

Old  Practice 

Two  furnaces,  6  men  to  each  furnace: 

One  furnace,     6  men,    910  shells  in  10  hours 
One  furnace,     6  men,  1207      "     "  " 
Total,  two  furnaces,  12  men,  2117      "     "   " 

Average  of  176  shells  per  man  per  10  hours. 


FIG.  149d. — Tempering  Furnaces — Discharge  End. 

New  Practice 
Two  furnaces,  with  3  men  total: 

One  furnace,  2508  shells  in  10  hours 

One  furnace,  2603      "     "   " 

Total,  two  furnaces,    3  men,   5111      "     "   "       " 
Average  of  1704  shells  per  man  per  10  hours. 

In  addition  to  obtaining  some  1000  per  cent,  increased  output  per  man 
with  the  new  underfired,  automatic  furnace,  it  has  been  shown  that 
in  the  old  method  the  average  rejections  were  running  around  15  to 
20  per  cent.,  and  were  only  about  3  per  cent,  for  the  new  practice. 


CARBON   STEELS 


253 


CARBON    STEELS    WITH    OVER   0.60   CARBON 

Treatment  in  General. — The  treatment  of  high-carbon  steel 
develops  into  the  two  propositions  of  hardening  and  annealing. 
Toughening,  as  referring  to  high  reheating  temperatures  subse- 
quent to  hardening,  is  but  very  little  used,  due  to  the  fact  that  these 
steels  are  too  brittle  for  ordinary  structural  purposes.  Similarly, 
tempering  is  governed  entirely  by  the  degree  of  hardness  required 
by  the  tool  and  is  dependent,  not  only  upon  the  chemical  analysis, 
but  in  a  larger  measure  upon  the  result  of  the  hardening  operation. 

Hardening. — The  precautions  to  be  adopted  in  hardening  may  be 
repeated  in  the  following  general  summary: 

(1)  Use  the  lowest  temperature  which  will  give  the  desired 

results. 

(2)  Heat  slowly  and  uniformly. 

(3)  The  higher  the  carbon  content,  the  greater  is  the  degree  of 

care  which  must  be  used,  and,  in  general,  the  more 
narrow  the  hardening  temperature  limits. 

The  temperatures  to  be  used  in  hardening  are  largely  governed 
by  the  carbon  content,  and  which,  in  turn,  influences  the  position 
of  the  critical  range.  We  may  sum  up  these  factors  as  follows: 


Carbon  Content.    Per  Cent. 

Critical  Range.     °F. 

Hardening  Temperature.  °F. 

0.60 

1340-1380 

1400-1460 

0.70 

1340-1375 

1400-1450 

0.80 

1340-1365 

1390-1450 

0.90 

1340-1360 

1375-1450 

1.00 

1340-1360 

1375-1450 

1.10 

1340-1360 

1375-1430 

1.20 

1340-1360 

1375-1430 

1.30 

1340-1360 

1375-1420 

1.40 

1340-1360 

1375-1420 

In  giving  the  above  hardening  temperatures  we  have  assumed 
that  the  previous  mechanical  and  heating  operations  have  left  the 
free  cementite  (in  hyper-eutectoid  steels — greater  than  0.9  per  cent, 
carbon)  well  distributed,  or  emulsified,  throughout  the  steel.  This 
will  generally  be  true  when  the  proper  finishing  temperatures, 
either  in  rolling  or  in  forging,  have  been  used.  In  such  cases, 
therefore,  it  will  not  be  necessary  to  heat  to  above  the  Ac. cm  range 
in  order  to  emulsify  the  free  cementite,  and  following  with  a  sub- 
sequent quenching  from  slightly  above  the  principal  critical  range. 


254 


STEEL  AND   ITS  HEAT  TREATMENT 


If,  however,  the  previous  heating  operations  have  left  the  free 
cementite  in  the  form  of  spines  or  network,  it  will  be  mandatory  to 
use  the  double-quenching  method  in  order  to  spheroidalize  this 
free  cementite  and  thus  obtain  the  maximum  wearing  and  cutting 
hardness;  for  details  of  such  procedure  the  subject  matter  in  Chap- 
ter VII  should  be  studied. 

Annealing. — The  general  subject  of  annealing  hyper-eutectoid 
steels  has  been  discussed  in  Chapter  III.  A  series  of  physical  test 
results  of  experiments  carried  out  by  Fdbry  1  upon  the  annealing  of 
steels  with  carbon  contents  of  0.58  to  1.36  per  cent.,  the  size  of  bar 
being  1.18  ins.  square,  and  the  selected  annealing  temperatures 
being  maintained  for  three  hours,  are  given  in  the  following  tables: 


0.58  PER  CENT.  CARBON  STEEL — ANNEALED 


Treat- 
ment. 

Tests. 

Hard- 
ness. 

Microscopic. 

Annealed 
Deg. 
Fahr. 

Tensile 
Strength, 
Lbs.  per 
sq.  In. 

Elastic 
Limit, 
Lbs. 
per 
sq.  In. 

Elonga- 
tion, 
per  cent 
In  3.15 
Ins. 

Red.  of 
Area, 
per  cent 

Brlnell 
No. 

Structure. 

Notes. 

1110 

99,540 

— 

15.8 

43.4 

196 

Free     ferrlte     and 
pearllte. 

Ferrlte  reticulated,  meshes 
filled  with  grainy  pearl- 
lte. 

1200 

98,420 

45,510 

17.7 

49.0 

183 

Ferrlte  begins  to  change 
Into  pearllte. 

1290 

84,200 

39,820 

20.7 

59.2 

174 

Smaller  ferrlte  crys- 
tals and  pearllte. 

Structure    essentially   dif- 
fering   from  other    speci- 
mens, because  the  ferrlte 
Is  uniformly  distributed. 

1380 

93,860 

36,980     18.6 

43.4 

176 

1470 

96,860 

39,820 

19.1 

36.8  . 

187 

Free     ferrlte     and 
pearllte. 

Ferrlte  forms  a  network; 
pearlite    partly    grainy, 
partly  lamellar. 

1560 

96,710 

39,820 

17.9 

35.6 

183 

1650 

98,130 

39,820 

18.6 

36.8 

185 

Network    of   large    ferrlte 
crystals  filled  with  pre- 
dominantly grainy  pearl- 
lte. 

1740 

93,860 

36,980 

16.7 

33.6 

187 

1830 

100,700 

39.820 

13.1 

25.2 

196 

Critical  range  Ac  commences  at  1337°,  maximum  at  1355°. 

1  Zs.  Fabry,  "  The  Variation  in  the  Mechanical  Properties  and  Structures  of  a 
Few  Special  Tool  Steels  Annealed  between  600°  and  1000°  C.  "  Int.  Soc.  Tes. 
Mat.,  1912. 


CARBON   STEELS 
0.81  PER  CENT.  CARBON  STEEL — ANNEALED 


255 


Treat- 
ment. 

Tests. 

Hard- 
ness. 

Microscopic. 

Annealed 
Deg. 
Fahr. 

Tensile 
Strength, 
Lbs.  per 
sq.  In. 

Elastic 
Limit, 
Lbs. 
per 
sq.  In. 

Elonga- 
tion, 
per  cent 
In  3.  15 
ins. 

Red.  of 
Area, 
per  cent 

Brlnell 

No. 

1110 

102,950 

— 

13.1 

37.6 

212 

1200 

106,400 

42,670 

14.0 

35.6 

207 

1290 

99,540 

39,820 

17.8 

43.4 

187 

1380 

100,700 

31,290 

14.6 

29.4 

183 

1470 

102,840 

31,290 

12.5 

14.8 

196 

1560 
1650 

105,250 

36,980 

13.1 

23.0 

187 

100,400 

31,290 

13.1 

19.4 

203 

1740 

98,980 

31,290 

10.2 

14.0 

207 

Critical  range  Ac  commences  at  1328°,  maximum  at  1337C 


0.92  PER  CENT.  CARBON  STEEL — ANNEALED 


Treat- 
ment. 

Tests. 

Hard- 
ness. 

M  Icroscopic. 

Annealed 
Deg. 
Fahr. 

Tensile 
Strength, 
Lbs.  per 
sq.  In. 

Elastic 
Limit, 
Lbs. 
per 
sq.  In. 

Elonga- 
tion, 
per  cent 
In  3.15 
Ins. 

Red.  of 
Area, 
per  cent 

Brlnell 
No. 

Structure. 

Notes. 

1110 

122,900 

— 

12.0 

23.0 

228 

Euctectlc. 

Grainy  pearllte  with  larger 
grains. 

1200 

120,600 

42,670 

13.0 

25.2 

217 

1290 

98,420 

36,980 

11.5 

33.6 

163 

Structure  perfectly  homo- 
geneous and  essentially 
differing  from  those  of 
other  specimens. 

1380 

91,030 

34,130 

17.8 

43.4 

174 

1470 

113,500 

31,290 

10.5 

14.8 

212 

Grainy  pearllte. 

1560 

112,100 

— 

9.1 

14.0 

207 

Lamellar  pearllte. 

1650 

112,500 

31,290 

8.7 

14.8 

216 

1740 

105,800 

31,290 

9.0 

11.6 

214 

1830 

123,450 

36,980 

6.8 

9.2 

228 

Indications  of  overheated 
structure. 

Critical  range  Ac  begins  at  1346°,  maximum  at  1355°. 


256 


STEEL  AND   ITS   HEAT  TREATMENT 


1.11  PER  CENT.  CARBON  STEEL — ANNEALED 


Treat- 
ment. 

Tes 

ts. 

Hard- 
ness. 

Microscopic. 

Annealed 
Deg. 
Fahr. 

Tensile 
Strength, 
Lbs.  per 
sq.  in. 

Elastic 
Limit, 
Lbs. 
per 
SQ.  In. 

Elonga- 
tion, 
per  cent 
in  3.15 
Ins. 

Red.  of 
Area, 
per  cent 

Brinell 
No. 

1110 

12°,  550 

— 

9.7 

20.8 

248 

1200 

126,300 

51,200 

12.6 

23.0 

235 

1290 

108,650 

54,040 

10.3 

23.0 

185 

1380 

88,180 

39,820 

19.6 

49.0 

170 

1470 

91,590 

36,980 

16.7 

36.8 

178 

1560 

96,700 

25,600 

10.3 

18.6 

196 

1650 

105,100 

29,580 

6.1 

10.0 

207 

1740 

100,700 

25,600 

6.6 

9.2 

202 

1830 

116,050 

36,980 

6.0 

6.8 

228 

Critical  range  Ac  begins  at  1337°,  maximum  at  1346°. 


1.36  PER  CENT.  CARBON  STEEL — ANNEALED 


Treat- 
ment. 

Tests. 

Hard- 
ness. 

Microscopic. 

Annealed 
Deg. 
Fahr. 

Tensile 
Strength, 
Lbs.  per 
sq.  in. 

Elastic 
Limit, 
Lbs. 
per 
sq.  in. 

Elonga- 
tion, 
per  cent 
in  3.15 
Ins. 

Red.  of 
Area, 
per  cent 

Brinell 
No. 

Structure. 

Notes. 

1110 

132,550 

— 

6.2 

11.6 

262 

Free  cementlte  and 
pearl  ite. 

Cementite  reticulated, 
meshes  filled  partly  with 
lamellar,  partly  with 
grainy  pearllte. 

1200 

129,700 

67,980 

8.5 

14.0 

255 

Free  cementlte  begins  to 
change  Into  pear  lite. 

1290 

123,450 

48,355 

9.6 

16.2 

288 

Fine-grained 
cementite. 

Structure  appears  uniform. 

1380 

93,300 

45,510 

14.6 

37.6 

192 

As  before,  grains  finer. 

1470 

90,310 

47,500 

17.3 

36.8 

187 

Free  cementlte  and 
grainy  pearllte. 

Cementite  concentrated 
again  into  small  crystals. 

1560 

93,300 

42,670 

13.7 

27.4 

187 

1650 

102,100 

32,710 

4.5 

5.0 

209 

Free  cementlte, 
grainy  and  lamel- 
lar pearllte. 

Cementlte  crystals  larger, 
pearlite  partly  in  lamellae. 

1740 

95,580 

28,446 

4.6 

6.8 

196 

Free  cementite  and 
and        lamellar 
pearllte. 

Structure  essentially  al- 
tered. Cementite  form;? 
a  network  with  large 
meshes.  Steel  Is  over- 
heated. 

1830 

101,830 

— 

2.6 

4.4 

223 

Critical  range  Ac  begins  at  1345°,  maximum  at  1355°. 


CHAPTER  XI 
NICKEL    STEELS 

Nickel  Steel. — Nickel  may  well  be  said  to  have  been  the  pioneer 
among  the  common  alloys  now  used  in  steel  manufacture.  Origi- 
nally added  merely  to  give  increased  strength  and  toughness  over 
that  obtained  in  the  ordinary  rolled  structural  steel,  the  development 
and  possibilities  of  heat  treatment  have  greatly  enhanced  its  value, 
so  that  nickel  steel  holds  a  premier  position  in  alloy  steel  metallurgy. 

The  chief  difficulties  attendant  upon  its  use  have  been  its  tend- 
ency to  develop  a  laminated  structure  and  its  liability  to,  seams. 
But  when  care  is  used  in  its  manufacture  and  rolling,  and  it  is  not 
made  in  too  large  heats  or  ingots,  and  when  piping  and  segregation 
are  avoided  by  confining  the  finished  product  to  that  produced  from 
the  bottom  two-thirds  of  the  ingot,  an  admirable  product  for  many 
purposes  is  obtained. 

Nickel  steel  has  remarkably  good  mechanical  qualities  when 
subjected  to  suitable  heat  treatment  and  is  an  excellent  steel  for  case 
hardening.  In  machining  qualities  it  usually  takes  first  place 
among  the  alloy  steels. 

Strength  and  Ductility. — Nickel  primarily  influences  the  strength 
and  ductility  of  steel  in  that  the  nickel  is  dissolved  directly  in  the 
iron  or  ferrite,  in  contradistinction  to  such  elements  as  chrome  and 
manganese  which  unite  with,  and  emphasize  the  characteristics  of 
the  cement  it  ic  component.  Thus,  for  the  forging  grades  of  ordinary 
nickel  steer  in  the  natural  condition,  the  addition  of  each  1  per  cent, 
of  nickel  up  to  about  5  per  cent,  will  cause  an  approximate  increase 
of  4000  to  6000  Ibs.  per  square  inch  in  the  tensile  strength  and 
elastic  limit  over  that  of  the  corresponding  straight  carbon  steel, 
without  any  decrease  in  the  ductility.  This  influence  of  nickel 
upon  the  static  strength  of  steel  also  increases  to  some  degree  with 
the  percentage  of  carbon.  To  illustrate  the  effect  of  nickel  upon 
steel  in  the  natural  condition:  a  steel  with  0.25  per  cent,  of  carbon 
and  3.5  per  cent,  nickel  will  have  a  tensile  strength  equivalent  to 
that  of  a  straight  carbon  steel  with  0.45  per  cent,  carbon,  a  propor- 

257 


258  STEEL  AND   ITS  HEAT  TREATMENT 

tionately  greater  elastic  limit,  and  the  advantageous  ductility  of  the 
lower  carbon  grade. 

Necessity  for  Heat  Treatment. — On  the  other  hand,  and  in  con- 
nection with  the  use  of  alloy  steels  in  general,  it  should  be  borne  in 
mind  that  such  steels  should  be  used  in  the  heat-treated  condition 
only — that  is,  not  in  either  an  annealed  or  natural  condition.  In 
the  latter  conditions  there  is  a  benefit,  as  compared  with  straight  car- 
bon steels  and  as  illustrated  above,  but  often  is  not  commensurate 
with  the  increased  cost.  In  the  heat-treated  condition,  however, 
there  is  a  very  marked  improvement  in  physical  characteristics. 
And  closely  associated  with  this  is  the  finely  divided  state  of  both 
ferrite  and  pear  lite  which  characterizes  heat-treated  nickel  steel. 

Nickel  vs.  the  Critical  Ranges. — One  of  the  most  interesting 
phenomena  connected  with  nickel  steel  is  the  effect  of  nickel  upon 
the  position  of  the  critical  ranges.  Nickel,  like  carbon,  has  the 
property  of  lowering  the  points  of  the  allot ropic  transformations  of 
iron,  but  in  a  more  marked  degree.  Just  as  we  have  seen,  in  the 
chapter  on  Hardening,  how  "  rapid  cooling  "  and  "  carbon  "  are 
"  obstructing  agents "  in  preventing  the  transformation  of  the 
austenite  into  martensite  into  pearlite,  so  likewise  does  nickel  act 
as  an  obstructing  agent.  The  effect  of  nickel  is  obtained  through 
a  lowering  of  the  Ar  ranges,  so  that  the  temperatures  of  the  critical 
ranges  on  cooling  may  even  be  brought  below  atmospheric  temper- 
atures. Thus  we  may  have  a  steel  which,  without  quenching,  may 
be  pearlitic,  troostitic,  martensitic  or  austenitic,  dependent  upon 
the  relative  percentages  of  nickel  and  carbon.  Hence,  such  steels 
containing  nickel  may  be  classified  according  to  their  microscopic 
constituents  which  are  obtained  upon  slow  cooling  from  a  high 
temperature. 

Classification  of  Nickel  Steels. — In  Fig.  150  there  is  shown 
graphically  the  influence  of  the  nickel-carbon  ratio  upon  the  struc- 
ture of  nickel  steels  as  cast,  or  as  moderately  cooled  from  a  high 
temperature. 

The  "  Pearlitic  "  nickel  steels  are  those  in  which  the  critical 
ranges  are  all  above  the  ordinary  temperatures,  so  that  such  steels 
as  slow  cooled  from  a  high  temperature  will  consist  of  pearlite 
plus  ferrite  (or  cementite).  These  are  the  ordinary  commercial 
nickel  steels,  and  are  represented  by  the  lower  triangle  of  Fig.  150. 

"  Martensitic  "  nickel  steels  contain  that  percentage  of  nickel 
and  carbon  which  will  so  lower  the  position  of  the  critical  ranges 
on  cooling  that  only  the  partial  transformation  may  proceed.  That 


NICKEL   STEELS 


259 


is,  the  austenite  is  transformed  into  martensite,  but  no  further — 
the  steel  being  too  rigid  to  allow  a  more  complete  transformation  at 
the  low  temperatures  involved.  These  steels  correspond  to  the  mid- 
dle triangle  in  Fig.  150.  Nickel  steels  martensitic  throughout 
have  no  practical  value,  as  it  is  impossible  to  work  or  machine  them. 
On  the  other  hand,  great  importance  is  attached  to  the  use  of  cer- 
tain pearlitic  nickel  steels  which  can  become  martensitic  upon  case 
carburizing — due  to  the  increased  carbon  content. 


20 


Austenltic 


Martensitic 


10 


Pearlitic 


0.40 


0.80 


1.20 


1.60 


FIG.  150. — Influence  of  the  Nickel-carbon  Content  upon  the  Structure  of  Nickel 

Steels  as  Cast. 


A  still  further  increase  in  the  nickel  or  carbon  content  will  cause 
the  critical  range  on  cooling  to  fall  below  atmospheric  temper- 
atures, so  that  such  steels  will  be  characterized  by  an  "  austenitic  " 
or  "  polyhedral  "  structure,  and  are  known  under  these  names. 

Micrographic  Structure.— These  changes  in  structure  are  illus- 
trated by  the  series  of  photomicrographs  (by  Savoia)  given  in  Figs. 
151  to  158,  all  the  steels  being  in  the  natural  condition,  having 
approximately  the  same  carbon  content  (0.25  per  cent.),  but  with 
increasing  percentages  of  nickel. 


260 


STEEL  AND   ITS  HEAT  TREATMENT 


FIG.  151.— Steel  with  0.25  per  cent.  Carbon,  2  per  cent.  Nickel.     X650. 

(Savoia.) 


»*^^^w 


aft* 

^5  *6s — 

L*$*1«»S 


FK;.  152. — Carbon,  0.25  per  cent.     Nickel,  5  per  cent.     X650. 

(Savoia.) 


NICKEL  STEELS 


261 


FIG.  153.— Carbon,  0.25  per  cent.     Nickel,  7  per  cent.     X650. 
(Savoia.) 


FIG.  154.— Carbon,  0.25  per  cent.     Nickel,  10  per  cent.     X650. 
(Savoia.) 


262 


STEEL  AND   ITS   HEAT  TREATMENT 


FIG.  155. — Carbon,  0,25  per  cent.    Nickel,  12  per  cent.     X650. 

(Savoia.) 


FIG.  156. — Carbon,  0.25  per  cent.     Nickel,  15  per  cent.     X650, 
(Savoia.) 


NICKEL  STEELS 


263 


FIG.  157. — Carbon,  0.25  per  cent.     Nickel,  20  per  cent.     X650. 

(Savoia.) 


FIG.  158.— Carbon,  0.25  per  cent.    Nickel,  25  per  cent.     X650. 
(Savoia.) 


264 


STEEL  AND   ITS  HEAT  TREATMENT 


It  will  be  seen  that  the  structure  of  the  2  per  cent,  nickel  steel 
(Fig.  151)  is  similar  to  that  of  a  corresponding  straight  carbon  steel, 
but  is  finer  and  more  homogeneous. 

The  5  per  cent,  nickel  steel  (Fig.  152)  shows  a  still  finer  and 
denser  structure,  in  that  the  pearlite  is  more  divided  and  distributed. 

With  the  7  per  cent,  nickel  (Fig.  153)  the  ferrite  and  pearlite 
are  still  seen,  but  they  are  distributed  in  a  special  manner  as  if 
disturbed  by  the  approach  of  a  transformation.  A  tendency  to 
orientiate,  somewhat  like  martensite,  is  also  noticeable. 


Pearlitic 


Martensitic 


Austenilic 


200,000 


150,000 


100,000 


FIG.  159. — Comparative  Physical  Properties  of  Nickel  Steels  with  0.25  per  cent. 

Carbon, 

The  steels  with  10  and  12  per  cent,  nickel  (Figs.  154  and  155) 
are  both  wholly  martensitic. 

With  the  15  per  cent,  nickel  (Fig.  156)  intensely  white  con- 
stituents appear  amidst  the  martensite  and  probably  represent  the 
first  appearance  of  austenite.  The  latter  increases  quite  noticeably 
in  the  steel  with  20  per  cent,  nickel  (Fig.  157),  taking  on  its  poly- 
hedral form. 

At  25  per  cent,  nickel  (Fig.  158)  the  whole  steel  is  characterized 
by  large  polyhedra  of  gamma-iron. 

Physical  Properties  with  Increasing  Nickel. — The  physical 
properties  of  these  same  cast  nickel  steels  are  plotted  graphically  in 


NICKEL  STEELS 


265 


the  chart  in  Fig.  159.  It  will  be  noted  that  the  abrupt  changes  in 
the  curves  correspond  very  closely  with  the  theoretic  structure  given 
by  the  upper  abscissae ;  and  that  these  same  physical  properties  are 
indicative  of  the  essential  characteristics  of  pearlite,  martensite  and 


F. 


1600 


1500 


1400 


1300 


1200 


\ 


£C.       0.20  0.40  0.60  0.80 

FIG.  160. — Critical  Changes  on  Heating  3  per  cent.  Nickel  Steel. 


1.0 


austenite,  as  denoted  by  the  tensile  strength,  ductility  (elongation) 
and  resistance  to  shock. 

Critical  Range  of  Pearlitic  Steels. — For  nickel  steels  with  less 
than  5  to  7  per  cent,  nickel,  each  1  per  cent,  nickel  lowers  the  crit- 
ical range  (Acl)  on  heating  by  about  15°  to  20°  F.,  and  also  lowers 
the  Arl  range  (cooling)  by  about  30°  to  40°  F.,  below  those  of  the 


266 


STEEL  AND   ITS  HEAT  TREATMENT 


corresponding  ranges  for  straight  carbon  steels  of  the  same  carbon 
and  manganese  contents.  Similarly,  there  is  a  corresponding  lower- 
ing of  the  other  critical  ranges. 

This  is  graphically  shown  in  Fig.  160,  which  the  author  has  care- 
fully plotted  from  a  series  of  observations  obtained  with  3  per  cent, 
nickel  steels  of  various  carbon  contents.  It  will  be  seen  from  this 
curve  that  the  critical  ranges  on  heating  are  about  60°  F.  below  the 
corresponding  straight  carbon  steels.  With  the  very  low  carbons 
there  appears  to  be  a  tendency  for  the  Ac3  curve  to  flatten  out;  this 
is  further  substantiated  by  results  with  steels  containing  5  to  7  per 
cent,  nickel.  Beyond  the  eutectoid  ratio  of  carbon  it  was  found 
that  the  Ar  range  would  begin  to  drop  quite  rapidly  (not  shown  in  the 
diagram)  below  its  normal  value,  as  might  be  expected  from  the 
fact  that  an  increase  in  carbon  in  these  steels  act  in  an  analogous 
manner  to  an  increase  in  nickel. 

The  approximate  temperatures  of  the  Acl  and  Ar  ranges  for 
nickel  steels  are  as  follows; 


Per  Cent.  Nickel.      Acl,  °  F. 

Ar,  °  F. 

0 

1340 

1280 

1.0 

1320 

1240 

2.0 

1300 

1200 

2.5 

1290 

1180 

3.0 

1280 

1160 

3.5 

1270 

1140 

4.0 

1260 

1120 

4.5 

1250 

1100 

5.0 

1240 

1080 

6.0 

1220 

1040 

7.0 

1200 

1000 

It  must  be  borne  in  mind,  however,  that  the  Acl  temperatures  may 
vary  considerably  from  steel  to  steel — but  those  given  above  will 
probably  be  about  the  average  of  those  obtained  in  practice,  and 
will  in  any  event  be  within  ±25°  F.  On  the  other  hand,  the  Arl 
temperatures  given  are  liable  to  an  even  greater  variation,  as  the 
maximum  temperature  attained  in  heating,  the  length  of  time 
occupied  in  both  heating  and  cooling,  the  effect  of  the  higher  car- 
bon contents,  and  many  other  experimental  factors,  all  tend  to 
change  the  position  of  the  Arl  range. 

From  these  figures,  and  from  the  critical  range  diagram  given 
for  3  per  cent,  nickel  steels,  it  will  be  observed  that  the  hardening  of 


NICKEL  STEELS  267 

nickel  steels  may  be  carried  out  at  temperatures  considerably  lower 
than  those  required  by  the  corresponding  straight  carbon  steels. 

The  Eutectoid  for  Nickel  Steels.— The  effect  of  additional 
nickel,  or  at  least  up  to  7  per  cent.,  is  to  reduce  the  eutectoid  carbon 
ratio  below  that  of  the  0.9  value  for  straight  carbon  steels.  That  is, 
a  nickel  steel  with  3  per  cent,  nickel  will  be  saturated,  having  neither 
excess  ferrite  nor  excess  cementite  (on  slow  cooling),  at  about  0.75 
to  0.8  per  cent,  carbon;  while  in  7  per  cent,  nickel  steel  the  eutec- 
toid ratio  appears  to  be  about  0.6  per  cent,  carbon.  This  fact  is  of 
great  importance  in  case-hardening  work,  in  that  it  not  only  permits 
of  a  shorter  duration  of  the  carburization  in  order  to  obtain  the 
maximum  carbon  concentration  necessary  in  the  case,  but  also 
reduces  the  carbon  content  over  which  it  is  likely  that  enfoliation 
may  occur. 

Heat  Treatment  of  Pearlitic  Nickel  Steels. — The  heat  treatment 
of  pearlitic  nickel  steels  presents  some  very  interesting  phenomena 
which  are  quite  distinctive  from  ordinary  straight  carbon  steels. 
One  would  naturally  assume  that  the  treatment  of  pearlitic  nickel 
steels  would  be  carried  out  in  an  analogous  manner  to  that  of  the 
ordinary  carbon  steels — that  is,  the  quenching  should  be  done  at  a 
temperature  slightly  in  excess  of  the  upper  critical  range,  provided 
that  the  duration  of  heating  at  the  maximum  temperature  has  been 
sufficient  to  effect  the  entire  solution  of  the  previous  components, 
together  with  their  diffusion  and  the  equalization  of  the  steel  as  a 
whole.  Similarly,  as  in  carbon  steels,  we  would  assume  that  we 
might  replace  the  length  of  heating  at  the  proper  quenching  temper- 
ature by  a  higher  temperature  in  order  more  quickly  to  effect  the 
equalization  of  the  steel;  provided,  however,  that  this  new  and 
more  elevated  temperature  shall  not  produce  too  great  a  deteriora- 
tion in  the  metal  through  increase  in  grain-size,  etc. — or  that  this 
higher  quenching  is  followed  by  a  quenching  at  the  proper  tempera- 
ture. In  straight  carbon  steels  the  change  of  structure  by  heating 
slightly  above  the  upper  critical  range  takes  place  quickly  as  a  general 
rule;  and  the  coarsening  or  embrittling  of  the  steel  also  occurs 
rapidly  when  higher  temperatures  are  used.  The  influence  of 
nickel  in  the  steel,  however,  often  necessitates  a  modification,  or 
permits  a  simplification,  of  these  general  principles,  both  in  regard 
to  the  temperature  of  quenching  and  the  length  of  heating. 

In  the  first  place,  the  addition  of  nickel  appears  to  make  the 
solution  of  the  ferrite  or  cementite  and  the  equalization  of  the  steel 
as  a  whole  take  place  more  slowly  than  in  the  ordinary  carbon 


268  STEEL  AND   ITS  HEAT  TREATMENT 

steels.  Thus,  if  we  take  a  steel  containing  some  4  or  5  per  cent, 
nickel,  and  a  mild  or  medium  carbon  content,  and  quench  it 
after  a  normal  heating  at  a  temperature  some  50°  F.  over  the 
critical  range,  the  transformation  is  often  incomplete  and  the 
martensite  not  uniformly  distributed  nor  equalized. 

In  such  an  event,  which  is  usually  characteristic  of  nickel  steel 
which  has  either  undergone  a  more  or  less  severe  elaboration  or  work- 
ing, or  has  been  finished  at  too  low  a  temperature,  or  has  been  sub- 
jected to  a  prolonged  heating  at  some  high  temperature,  there  are 
then  four  methods  of  procedure  available: 

(1)  Prolonged  heating  at  the  proper  quenching  temperature 

to  effect  the  necessary  transformation,  followed  by 
quenching; 

(2)  Heating  to  a  higher  temperature  than  in  (1),  and  quench- 

ing; 

(3)  Heating  to  the  higher  temperature,  cooling  to  a  temper- 

perature  a  little  above  the  Ar  temperature,  and  then 
quenching; 

(4)  Quenching,  or  air  cooling,  from  the  higher  temperature, 

followed  by  a  normal  reheating  to  a  temperature  slightly 
hi  excess  of  the  Ac  range,  and  quenching. 

These  propositions  at  once  evoke  a  discussion  of  further  char- 
acteristics which  the  presence  of  nickel  involves. 

If  an  ordinary  carbon  steel  be  heated  for  a  considerable  duration 
of  time  at  a  temperature  even  slightly  over  the  critical  range,  the 
grain-size  will  begin  to  increase,  with  a  corresponding  decrease  in 
both  the  static  and  dynamic  strength  of  the  material.  On  the 
other  hand,  if  a  nickel  steel  be  subjected  to  a  length  and  temperature 
of  heating  equivalent  to  that  of  the  carbon  steel,  the  pearlite  and 
ferrite  grains  will  remain  (after  slow  cooling)  considerably  finer, 
more  uniformly  distributed,  and  much  more  subdivided  than  the 
carbon  steel.  This  characteristic  permits  the  greater  duration  of 
heating  as  required  under  the  first  proposition,  without  any  per- 
ceptible deterioration  such  as  would  be  noticeable  in  a  straight  car- 
bon steel  with  the  same  prolonged  heating.  However,  such  treat- 
ment— when  required — is  disadvantageous  from  the  commercial 
standpoint,  as  it  decreases  the  capacity  of  the  heat  treatment  plant, 
with  a  corresponding  increase  in  the  cost  of  production. 

Again,  the  increased  brittleness  due  to  a  more  or  less  prolonged 
heating  at  temperatures  in  excess  of  the  upper  critical  range  is  con- 
siderably less  for  nickel  steels  than  for  ordinary  carbon  steels.  This 


NICKEL  STEELS  269 

fact  is  well  illustrated  by  the  following  results  upon  a  straight  carbon 
steel  in  comparison  with  a  2  per  cent,  nickel  steel  of  the  same  carbon 
content,  taken  from  a  memoire  l  by  Guillet: 


Length  of  Heating 

Resistance 

to  Shock. 

at  1830°  F. 

Ordinary  extra-soft  steel. 

Extra-soft  steel  with 
2  per  cent  nickel. 

Normal  heating 

20      kgms. 

60  kgms.  (not  broken) 

Four  hours 

4  .  5  kgms. 

60  kgms.  (not  broken) 

Six  hours  

4.0  kgms. 

60  kgms.  (not  broken) 

If,  in  order  to  obtain  the  full  equalization  of  the  steel  and  also  to 
avoid  a  prolonged  heating  at  the  lower  and  theoretic  temperature, 
it  shall  be  necessary  to  heat  and  quench  from  a  higher  temperature, 
such  operation  may  be  undertaken  without  that  fear  of  greatly 
increasing  the  brittleness  which  would  most  probably  occur  in  a 
straight  carbon  steel. 

Although  it  is  granted  that  a  heating  to  this  high  temperature 
may  be  necessary,  a  quenching  from  this  same  high  temperature 
would  not  be  entirely  logical  if  this  were  to  be  the  only  quenching, 
and  also  if  viewed  from  the  standpoint  of  the  best  product.  In 
such  high  temperature  quenchings  there  is  the  ever-present  danger 
of  cracking  and  warping.  Further,  it  is  a  generally  admitted  fact 
that  no  change  in  the  molecular  arrangement  of  the  steel  occurs 
in  cooling  such  a  steel  until  the  upper  critical  range  on  cooling  (Ar3) 
is  reached.  Assuming  this  to  be  true,  we  may  then  modify  the  previ- 
ous treatment  (proposition  2)  by  first  cooling  the  steel — after  heat- 
ing to  the  high  temperature — to  a  temperature  slightly  above  that 
of  the  Ar3  range,  and  then  quench,  as  stated  under  proposition  3. 
This  treatment  will  retain  all  the  benefits  which  may  accrue  from 
the  original  high-temperature  heating,  but  at  the  same  time  will 
diminish  to  a  considerable  degree  the  dangers  of  cracking  and 
warping.  And  as  the  critical  ranges  on  cooling  in  nickel  steels  are 
even  further  below  those  of  the  Ac  ranges  in  comparison  with 
straight  carbon  steels,  this  quenching  temperature  will  be  reason- 
ably low. 

Objections  which  may  be  offered  to  this  method  are  that  the 
quenching  from  just  over  the  Ar  range  may  not  give  the  maximum 

*M.  L.  Guillet,  "  Traitements  thermiques  des  aciers  speciaux,"  Rev.  de 
Met.,  July,  1910. 


270  STEEL  AND  ITS  HEAT   TREATMENT 

hardening  effect  unless  the  quenching  temperature  has  been  gauged 
just  rightly,  or  if  the  carbon  content  is  low.  The  first  objection  may 
be  overcome  by  suitable  temperature  control;  if  the  quenching 
temperature  should  fall  too  low,  the  difference  in  the  hardening 
effect,  for  forgings  or  full-heat  treated  work,  may  be  later  corrected 
by  using  a  lower  toughening  or  drawing  temperature.  By  the  use 
of  exact  methods,  such  as  one  furnace  maintained  at  the  high  tem- 
perature, and  then  another  furnace  (into  which  the  steel  may  be 
subsequently  placed)  maintained  at  the  temperature  a  little  over  the 
Ar  range,  the  first  objection  may  be  entirely  cleared  away.  The 
second  objection  may  also  be  at  once  overruled  by  the  fact  that 
the  treatment  of  the  low-carbon  steels  is  generally  limited  in  com- 
mercial work  to  the  obtaining  of  a  suitable  toughness  and  absence 
of  brittleness  (regeneration),  and  that  it  is  usually  not  desired  to 
obtain  the  maximum  hardness. 

In  brief,  it  does  not  matter  whether  the  same  mechanical  prop- 
erties in  a  pull  test  are  obtained  by  a  quenching  made  at  a  very 
high  temperature,  or  by  a  quenching  at  a  lower  temperature  follow- 
ing the  return.  As  these  results  in  the  mechanical  properties  are 
practically  the  same,  the  treatment  under  proposition  3,  as  compared 
with  Number  2,  is  always  more  advantageous  from  the  point  of 
view  of  non-brittleness  and  probably  also  from  the  point  of  view  of 
the  strength  of  the  piece. 

The  most  serious  objection  to  the  treatment  in  either  (2)  or  (3), 
however,  is  the  increase  in  brittleness  which  is  liable  to  occur  if 
the  high  temperature  heating  is  unduly  prolonged.  Although  the 
presence  of  nickel  tends  to  diminish  such  a  condition,  the  effect 
of  high  heating  is  always  towards  the  increase  in  grain-size,  and 
coarse  martensite  generally  corresponds  to  a  diminution  in  the 
strength  of  the  steel.  Assuming  that  a  temperature  considerably 
in  excess  of  the  upper  critical  range  is  mandatory,  any  ill  effects 
resulting  therefrom  may  be  entirely  overcome  by  a  double  heating 
and  cooling,  and  yet  also  retaining  the  benefits  of  such  high  temper- 
ature heating.  That  is,  by  cooling  the  steel — but  not  quenching, 
unless  the  original  structure  is  very  bad  indeed;  or  unless  the  most 
perfect  structure  is  desired — from  the  high  temperature  to  a  tem- 
perature under  that  of  the  Al  range,  in  order  to  impress  the  effect 
of  the  high  temperature  upon  the  steel,  followed  by  a  reheating 
to  a  temperature  slightly  in  excess  of  the  upper  critical  range,  and 
then  quenching.  Such  a  hardening  treatment,  either  with  air  cool- 
ing and  a  subsequent  single  quenching,  or  with  a  double  quenching, 


NICKEL  STEELS  271 

is  the  best,  although  the  most  expensive.  As  this  method  has  been 
discussed  in  its  relation  to  carbon  steels,  and  as  its  influence  is 
approximately  the  same  with  pearlitic  nickel  steels,  it  will  not  be 
necessary  to  dwell  further  upon  it. 

In  general,  the  treatments  (for  the  best  quenching  effect)  given 
under  (1),  (2)  and  (3)  will  suffice  for  ordinary  commercial  practice, 
but  with  the  preference  given  to  (1)  or  (3).  That  under  (4)  is  best 
if  the  higher  cost  is  permissible. 

Moreover,  it  should  be  borne  in  mind  that,  in  perhaps  even  a 
majority  of  cases,  the  regular  and  normal  quenching  from  a  tem- 
perature slightly  in  excess  of  the  upper  critical  range  (Ac3),  after 
a  thorough  and  uniform  heating  at  that  temperature,  will  generally 
suffice — and  especially  for  small  work.  But  in  order  more  fully  to 
explain  the  difficulties  which  are  sometimes  met  with  in  the  treat- 
ment of  nickel  steels,  the  author  has  entered  into  the  foregoing 
explanations.  As  a  safe  and  general  fundamental  principle,  re- 
peatedly urged,  it  is  always  advisable  to  quench  from  the  lowest 
temperature  which  will  give  the  desired  results. 

The  tempering  and  toughening  of  pearlitic  nickel  steels  is  carried 
out  exactly  as  with  straight  carbon  steels,  previously  explained,  and 
is  dealt  with  in  more  detail  later  on  in  the  chapter. 

CARBURIZATION    OF    NICKEL    STEELS 

The  general  principles  of  the  carburization  of  nickel  steels  are 
similar  to  those  which  apply  to  straight  carbon  steels,  and  should 
not  require  repetition.  There  are,  however,  certain  peculiarities, 
presenting  both  advantages  and  disadvantages,  which  should  be 
mentioned. 

(1)  Nickel  steels  show  less  susceptibility  to  brittleness  due  to 
prolonged  heating  at  the  high  temperatures  often  used  in  carburiza- 
tion than  do  the  corresponding  carbon  steels.     This  important  fact 
not  only  gives  a  steel  better  able  to  withstand  shock,  but  also  gives 
a  well-defined  means  of  simplifying  the  subsequent  heat  treatment 
if  desired.     Such  advantages  may  be  readily  obtained  by  the  addi- 
tion of  even  2  per  cent,  of  nickel,  and  largely  compensate  for  the 
slightly  higher  cost. 

(2)  The  variations  in  the  concentration  of  the  carbon  in  the 
carburized  zones  are  more  gradual  and  uniform  in  nickel  steels  than 
in  carbon  steels.     This  better  distribution  of  the  carbon  therefore 
tends  towards  the  prevention  of  a  distinct  line  of  demarkation  be- 
tween the  different  zones,  and  thus  to  eliminate  the  chipping  and 


272 


STEEL  AND   ITS  HEAT  TREATMENT 


flaking  off  of  the  case.     Similarly,  the  phenomenon  of  "  liquation, "- 
a  principal  factor  in  such  enfoliation — is  less  marked,  under  equal 
conditions,  in  nickel  steels  than  in  carbon  steels. 

(3)  Although  it  is  true  that  carburization  proceeds  with  greater 
slowness  with  some  solid  carburizing  compounds,  referring  to  their 
use  with  nickel  steels  with  less  than  say  3.5  per  cent,  nickel,  the  use 
of  a  mixed  cement  (carbon  monoxide  plus  carbon)  will  effect  a  car- 
burization with  a  rapidity  equal  to  that  with  ordinary  carbon  steels. 

(4)  Under  the  same  conditions,  the  depth  of  penetration  of  the 
carburized  zone  for  a  given  time,  using  a  mixed  cement,  is  even 
slightly  higher  for  nickel  steels  than  for  carbon  steels. 

(5)  The  lesser  hardness  which,   with  the  same  treatment,   is 
possessed  by  the  carburized  zones  in  nickel  steel  as  compared  with 
the  carburized  zones  in  carbon  steels  under  identical  conditions,  is 
due  not  only  to  the  different  effects  of  different  quenchings,  but 
also  to  the  smaller  concentration  (especially  for  less  than  3  per  cent, 
nickel)  of  carbon  in  the  carburized  zones.     This  disadvantage  may 
be  eliminated  by  raising  the  carbon  in  the  carburized  zone  by  a 
suitable  change  in  the  conduct  of  the  carburization. 

(6)  When  the  nickel  content  exceeds  3  per  cent,  the  maximum 
concentration  of  the  carbon  in  the  carburized  zones  decreases  with 
an  increase  in  the  percentage  of  nickel  contained  in  the  steel.     The 
following  table,  from  Giolitti,1  contains  data  relative  to  the  maximum 
concentration  reached  by  the  carbon  in  the  carburized  zones  when 
carburizing,  under  various  conditions,  steels  with  varying  nickel 
contents : 


Condition  of  Carburization. 

Nickel  Content. 

2% 

3% 

5% 

25% 

30% 

Carbon  monoxide: 
5  hours  at  1740°  F  

0.38 
0.35 

1.53 

0.23 
0.35 

0.93 

1.28 

0.70 
0.80 
0.73 
0.74 
0.83 

0.15, 
0.17 

0.39 
0.63 

0.67 
0.40 

5  hours  at  1920°  F  
Ethylene: 
5  hours  at  1740°  F  

1.12 

5  hours  at  1920°  F  

0.84 

0.64 
0.59 

0^73 

Mixed  cement: 
2  hours  at  1830°  F  

0.70 
1.12 
0.83 
0.92 
1.07 

5  hours  at  1830°  F. 



5  hours  at  1920°  F 

2  hours  at  2010°  F  
5  hours  at  2010°  F  

1  F.  Giolitti,  "  The  Cementation  of  Iron  and  Steel.' 


NICKEL  STEELS 


273 


(7)  By  employing  a  nickel  steel  of  the  proper  nickel  content,  and 
carburizing  in  such  a  manner  as  to  attain  a  definite  maximum 
carbon  concentration  in  the  case,  a  steel  characterized  by  a  tough 
core  and  a  martensitic  structure  in  the  case  may  be  obtained  with- 
out subsequent  quenching.  The  approximate  maximum  carbon  con- 
centration in  the  case  which  it  is  necessary  to  obtain  for  steels  with 
definite  percentages  of  nickel  in  order  to  produce  a  martensitic  struc- 
ture without  quenching,  may  be  given  about  as  follows: 


Per  cent.  Nickel. 

Per  cent.  Carbon. 

Per  cent.  Nickel. 

Per  cent.  Carbon. 

2 

1.50 

5 

0.95 

3 

1.30 

6 

0.85 

4 

1.10 

7 

0.75 

Such  methods  eliminate  the  necessity  for  subsequent  heat  treat- 
ment, if  so  desired,  and  effect  corresponding  reductions  in  the  cost 
of  the  process,  besides  obviating,  in  a  large  measure,  such  important 
factors  as  warping,  grinding,  etc.  Further,  by  extending  the  car- 
burization  so  as  to  reach  a  maximum  of  1.5  per  cent,  carbon  at  the 
periphery,  for  steels  containing  5  to  7  per  cent,  nickel,  there  can 
also  be  obtained  a  superficial  layer,  superimposed  upon  the  mar- 
tensitic zone,  containing  austenite,  which  easily  admits  of  polishing 
without  loss. 

(8)  The  lower  temperatures  at  which  the  critical  ranges  are 
located,  in  the  pearlitic  nickel  steels,  permit  a  lower  temperature 
to  be  used  in  case  carburizing,  which  is  an  important  factor  in 
intricate  or  exact  work. 


THERMAL    TREATMENT  AFTER   CARBURIZATION 

In  general,  the  thermal  treatment  of  nickel  steels,  subsequent 
to  case  carburizing,  may  be  classified  according  to  the  structure  of 
the  case  after  slow  cooling  from  the  temperature  of  carburization — 
that  is,  whether  it  is  pearlitic  or  martensitic.  Although  this  struc- 
ture depends  primarily  upon  the  conduct  of  the  carburization  and 
the  maximum  carbon  concentration  thus  obtained  in  the  case,  the 
procedure  as  practically  carried  out  in  commercial  work  will  usually 
give  (upon  slow  cooling  after  carburization)  (1)  a  pearlitic  structure 
in  the  case  for  steels  with  nickel  contents  under  4  per  cent,  and  (2)  a 
more  or  less  martensitic  case  for  steels  with  4  to  7  per  cent,  nickel. 


274 


STEEL  AND   ITS  HEAT  TREATMENT 


As  explained  in  Chapter  VII,  the  best  treatment  which  can  be 
given  any  case-carburized  pearlitic  steel  is  that  involving  a  double 
quenching.  Each  quenching — for  regeneration  and  for  hardening — 
should  be  carried  out  at  the  most  suitable  temperature,  and  which  is 
fixed  by  the  transformation  points  of  the  core  and  case  respectively. 
These  treatments  for  nickel  steels  with  2  to  2J  and  3  to  3J  per  cent, 
nickel  are  approximately  as  follows: 

Carburization. — Carburize  at  the  desired  temperature,  usually 
1600°  to  1750°  F.  Cool  slowly  in  the  carburizing  material 
(assuming  solid  cements). 

Thermal  Treatment. — 


Nickel  Content,  Per  cent. 

2  to  '* 

l|. 

3  t( 

)  3*. 

Carbon  Content,  Per  cent. 

0.10  to  0.15 

0.15  to  0.20 

0.10  to  0.15 

0.15  to  0.20 

Regenerative 
quenching  

(a)  1550-1600° 

1500-1550° 

1475-1525° 

1450-1500° 

Hardening 
quenching 

or  (6)  1600° 
1325-1375° 

1600° 
1325-1375° 

1600° 
1300-1350° 

1600° 
1300-1350° 

The  steel  may  be  removed  from  the  quenching  bath  as  soon  as 
it  loses  its  red  color  during  the  regenerative  quenching,  and  imme- 
diately reheated  for  the  second  quenching.  Practice  differs  as  to 
the  temperature  to  be  used  for  the  regenerative  quenching,  some 
preferring  to  quench  from  a  temperature  slightly  above  the  Ac3 
range,  as  under  (a),  while  others  prefer  to  use  the  higher  temper- 
ature (6).  The  reasons  for  these  have  been  discussed  in  previous 
sections. 

On  the  other  hand,  for  2  per  cent,  nickel  steels,  Guillet  recom- 
mends temperatures  distinctly  higher  than  those  given  above — 
which  probably  coincide  with  the  best  American  practice — for  the 
regenerative  quenching,  and  which  he  gives  as  follows: 

Regenerative  quenching 1760°  to  1800° 

Hardening  quenching 1365°  to  1420° 

The  hardening  quenching  should  be  conducted  at  the  lowest  pos- 
sible temperature  at  which  the  metal  of  the  case  will  become  glass- 
hard.  In  many  instances  it  will  be  found  that  temperatures  some- 
what lower  than  those  given  in  the  above  table  can  be  used.  For 


NICKEL  STEELS  275 

example,  the  critical  curve  given  in  Fig.  161  for  a  steel  with  0.13  per 
cent,  carbon,  0.49  per  cent,  manganese  and  3.35  per  cent,  nickel, 
shows  the  Acl  range  to  be  about  1250°  F.,  so  that  a  temperature 
under  1300°  to  1350°  might  be  used  effectively  for  the  hardening 
quenching. 

The  effect  of  different  treatments  upon  Quillet's  2  per  cent, 
nickel  steel  in  its  resistance  to  shock  is  shown  in  the  following  table: 

Treatment.  Resistance  to  Shock. 

Steel  with  2  per  cent,  nickel  and  0.1  per  cent,  carbon:    • 

Heated  to  1700°  F.  and  air  cooled 33.4  kgms. 

Quenched  in  water  from  1700°  F 34 . 5 

Same  steel  cemented  at  1830°  F.  for  1.2  mm.: 

Slow  cooled 31 .0 

Quenched  in  water  from  1830°  F 33.5 

Double-quenched  in  water,  1830°  and  1380° 36.0 

Quenched  in  water  from  1380°  F , , , , , ,  32, 0 


FIG.  161, — Critical  Range  Diagram. 

Simplified  Thermal  Treatments  after  Carburization. — On  account 
of  the  fact  that  the  brittleness  of  the  core  (with  nickel  steels)  is  not 
greatly  increased  by  the  heating  during  carburization  if  the  tempera- 
ture of  that  operation  is  not  too  high,  and  as  the  Ac3  range  of  the 
ordinary  nickel  steels  is  considerably  lower  than  that  of  the  corre- 
sponding straight  carbon  steel,  it  makes  it  possible,  as  we  have  seen, 
to  effect  a  regenerative  quenching  at  a  temperature  in  the  neighbor- 
hood of  1500° -1550°  F,  Further,  as  the  nickel  steel  case  can  be 


276  STEEL  AND   ITS  HEAT  TREATMENT 

hardened  at  a  temperature  considerably  above  the  normal  Acl 
without  losing  too  much  of  its  hardness  or  increasing  too  largely  in 
brittleness,  it  follows  that,  in  many  instances,  the  regenerative 
quenching  may  also  serve  as  a  hardening  quenching.  This  permits 
the  simplification  of  the  treatment  to  a  single  quenching  for  nickel 
steels,  unless  the  piece  is  to  be  subjected  to  exceptional  stress.  The 
practical  usefulness  of  this  method  is  obvious. 

It  is  evident,  however,  that  the  double  quenching  will  always 
give  considerably  better  results  for  both  core  and  case.  This  is 
particularly  shown  in  the  depth  and  degree  of  hardness  obtained  by 
the  lower  quenching  over  the  higher  quenching  temperature  by  the 
following  experiments  by  Guillet  on  2  per  cent,  nickel  steels: 


Shore  Hardness  Numbers. 

Treatment. 

Maximum. 

Minimum. 

Mean. 

Cemented  pieces,  not  quenched  

40 

39 

39.37 

Cemented  pieces,  quenched  from  1830°  F. 

84 

69 

78.05 

Cemented  pieces,  quenched  from  1380°  F. 

88 

85 

86.56 

Case  Hardening  by  Air  Cooling. — Again,  the  case-hardening 
process  may  be  even  further  simplified  by  the  use  of  nickel  steels  with 
3.5  per  cent,  of  nickel,  or  more,  and  conducting  the  carburization  in 
such  a  manner  as  will  produce  a  maximum  carbon  content  in  the  case, 
which  will  give  a  martensitic  structure  on  air  cooling  from  the  tem- 
perature of  carburization.  An  example  of  this  is  shown  in  Figs.  162 
and  163,  representing  a  case-carburized  steel  with  an  initial  carbon 
content  of  0.176  per  cent.,  with  3.44  per  cent,  nickel;  the  steel  was 
then  air  cooled  directly  after  carburization.  The  thickness  of  the 
martensitic  zone  is  about  0.5  mm.  Under  the  lower  magnification 
(Fig.  163)  a  solid  troostitic  band  is  seen  to  separate  the  martensite 
and  the  sorbito-pearlite  portions.  The  principal  advantage  which 
this  method  presents  consists  of  its  great  simplicity,  and  also  in  the 
fact  that  it  permits  the  avoidance  of  deformation  which  so  often 
accompanies  any  quenching  operation.  Nickel  steels  which  are 
martensitic  after  air  cooling  may  be  troostitic,  sorbitic,  or  even  pearl- 
itic  after  very  slow  cooling  in  the  furnace,  while  they  may  be  austen- 
itic  on  water  quenching. 

Case  Hardening  5  to  7  Per  Cent.  Nickel  Steels.— Advancing 
another  step  and  using  nickel  steels  with  5  to  7  per  cent,  nickel,  we 
find  that  the  ordinary  carburization  and  subsequent  slow  cooling 


NICKEL  STEELS 


277 


FIG.  162.— Nickel  Steel.     Nickel,  3.44  per  cent.     Carbon,  0.176  per  cent. 
Case  Carburized  and  Air  Cooled.     X 100.     (Sauveur  and  Reinhardt.) 


FIG.  163. — Same  Steel  and  Treatment  as  in  Fig.  162. 
(Sauveur  and  Reinhardt.) 


X50. 


278 


STEEL  AND   ITS  HEAT  TREATMENT 


will  produce  a  case  with  characteristics  varying  over  a  wide  range, 
dependent  upon  the  nickel-carbon  ratio  in  the  case.  In  other  words, 
the  transformation  range  of  the  metal  of  the  case  on  cooling  may  be 
even  further  reduced  below  that  of  the  previous  example,  giving 
either  a  martensitic  or  martenso-austenitic  structure  upon  slow  cool- 
ing. Therefore,  when  it  is  not  required  to  produce  an  extremely 
tough  core,  nor  to  obtain  extreme  hardness  in  the  case,  the  carbur- 
ized  pieces  with  5  to  7  per  cent,  nickel  may  simply  be  allowed  to 
cool  slowly  in  the  carburizing  mixture  after  carburization. 

The  use  of  the  method  just  indicated,  however,  has  its  dis- 
advantages. The  following  table  shows  the  results  of  scleroscope 
hardness  tests  made  by  Guillet  on  a  steel  containing  7  per  cent, 
nickel  and  0.12  per  cent,  carbon,  carburized  to  a  depth  of  0.1  mm., 
but  not  quenched: 


Treatment  and  Tests. 

Shore  Hardness  Numbers. 

Mean. 

Maximum. 

Minimum. 

Test  made  on  the  surface. 

18.5 
26.5 
24.5 
20.2 

21 
27 
25 
20 

17 
26 
24 
21 

Test  made  at  a  depth  of  0.2  mm  

Test  made  at  a  depth  of  0.4  mm  ....... 
Test  made  at  a  depth  of  0.6  mm  

From  this  particular  instance  it  will  be  seen  that  the  surface  zone 
is  partly  austenitic,  so  that  a  very  great  hardness  is  not  obtained. 

In  the  second  place,  it  is  evident  that  the  core  of  the  piece  thus 
treated  has  not  been  regenerated,  although,  as  we  have  said  before, 
nickel  steel  does  not  have  the  maximum  brittleness  which  a  straight 
carbon  steel  would  have  under  the  same  conditions  of  cooling. 

The  structure  of  a  steel  containing  4.86  per  cent,  nickel  and  0.115 
per  cent,  carbon,  intensely  carburized,  and  air  cooled,  is  shown  in 
Fig.  164.  This  photomicrograph  shows  that  the  effect  of  such 
treatment  is  to  produce  a  case  which  is  largely  austenitic. 

The  best  practice,  however,  both  American  and  foreign,  specifies 
the  use  of  a  double-quench  treatment  subsequent  to  a  mild  carburiza- 
tion, and  using  a  soft  steel  with  4.5  to  6  per  cent,  nickel.  Such  steels 
have  many  peculiar  advantages:  the  carburization  may  be  con- 
ducted at  a  moderate  temperature;  a  maximum  carbon  content  in 
the  carburized  zone  of  only  about  0.45  to  0.6  per  cent,  is  necessary 
to  produce  a  glass-hard  surface  on  oil  quenching;  and  the  core 
becomes  exceedingly  strong,  as  well  as  tough  and  non-brittle.  From 


NICKEL  STEELS  279 

these  facts  it  is  evident  that  the  lowering  of  the  maximum  carbon 
concentration  to  a  percentage  not  exceeding  that  of  the  eutectoid 
ratio  will  almost  entirely  eliminate  the  danger  of  chipping  and 
flaking.  The  use  of  moderate  temperatures  for  carburization  and 
of  oil  for  quenching  decreases  the  liability  to  warping  and  fracture. 
The  physical  characteristics  of  the  carburized  zone  after  the  second 
oil  quenching,  the  steel  of  the  case  having  an  approximate  chemical 


FIG.  164. — Nickel  Steel.     Nickel,  4.86  per  cent.     Carbon,  0.115  per  cent. 
Case  Carburized  and  Air  Cooled.     XlOO,     (Sauveur  and  Reinhardt.) 


composition  of  0.45  per  cent,  carbon  and  5.0  per  cent,  nickel,  will 
be  approximately: 

Tensile  strength,  Ibs.  per  sq.  in 260,000 

Elastic  limit,  Ibs.  per  sq.  in 250,000 

Elongation  in  2  ins.,  per  cent 2 

Reduction  of  area,  per  cent 5 

Brinell  hardness 490 

Scleroscope  hardness 74 


280  STEEL  AND  ITS  HEAT  TREATMENT 

The  following  physical  results  taken  from  the  core  of  a  double- 
quenched  steel  analyzing: 

Carbon.. 0.105 

Manganese 0 . 43 

Phosphorus 0. 014 

Sulphur 0.030 

Silicon 0.11 

Nickel 5.0 

show  that  the  core  will  have  great  strength,  high  ductility  (from  the 
reduction  of  area) ,  and  very  slight  brittleness  (as  shown  by  the  shock 
test) : 

Tensile  strength,  Ibs.  per  sq.  in 200,000 

Elastic  limit,  Ibs.  per  sq.  in 170,000 

Elongation  in  2  ins.,  per  cent 12 

Reduction  of  area,  per  cent 54 

Resistance  to  shock 75 

Brinell  hardness 295 

The  same  steel,  having  an  upper  critical  range  of  1425°  F.,  and 
annealed  at  1475°  F.,  gave: 

Tensile  strength,  pounds  per  square  inch. . .  .  90,600 

Elastic  limit,  pounds  per  square  inch 60,160 

Elongation  in  2  ins.,  per  cent 20 

Reduction  of  area  per  cut 60. 5 

Resistance  to  shock 116 

Brinell  hardness .        179 

The  regenerative  quenching  for  these  steels  may  be  carried  out 
either  at  a  temperature  slightly  in  excess  of  the  upper  critical  range — 
-  or  at  about  1475°  F.,  or,  in  order  to  more  fully  equalize  and  effect 
the  regeneration  of  the  core,  at  some  higher  temperature,  such  as 
1600°  F.  The  hardening  quenching  temperature  should  be  slightly 
over  the  Ac  range  of  the  case,  or  approximately  1275°  to  1325°  F. 
Oil  may  be  used  for  both  quenchings.  For  the  characteristic  French 
steel  containing  about  6  per  cent,  nickel  Guillet  recommends  the 
temperatures  of  1560°  and  1250°  F.  respectively  for  the  double 
quenching. 

3.5    PER    CENT.    NICKEL   STEEL 

We  have  previously  discussed  some  of  the  factors  which  enter 
into  the  quenching  of  nickel  steels  in  general.     Whether  or  not  it 


NICKEL  STEELS 


281 


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282 


STEEL  AND   ITS  HEAT  TREATMENT 


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NICKEL  STEELS 


283 


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284 


STEEL  AND   ITS  HEAT  TREATMENT 


(9)  jaqranji  ssaapJUR  adoosoaops  3-iou.s 
§  S  § 


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ical  Ranges  :  Size 
Ac  1240°  1  in 
Ar  1060° 


ical  Analysis 
C.  0.39 
Mn.  0.48 
P.  0.009 
S.  0.023 
Si.  0.10 
Ni.  3.36 


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NICKEL  STEELS 


285 


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STEEL  AND   ITS  HEAT  TREATMENT 


may  be  necessary  to  use  a  temperature  considerably  in  excess  of  the 
upper  critical  range  for  hardening  will  depend  upon  the  condition  of 
the  steel  as  it  comes  to  the  heat-treatment  plant;  in  many  cases  the 
normal  heating  and  quenching  will  suffice  for  general  purposes. 
The  normal  hardening  temperatures  for  3  to  3J  per  cent,  nickel  steel 
may  be  approximately  determined  by  reference  to  the  critical  range 
diagram  in  Fig,  160,  and  by  adding  50  to  100  degrees  to  the  upper 


500 

1 

IS   400 

"3 

a 


300 


20U 


A 


Effect  of  Mass 
C.  0.25  P.  0.01 

Si.  0.09  S.  0.012 

Mn.  0.67  Ni.  3.47 


*'/* 


Size  in  Inches 


FIG.  170.— Effect  of  Mass  upon  the  Hardness  of  Nickel  Steel,  Oil  Treated. 
(Matthews  &  Stagg.) 


critical  range  for  the  given  carbon  content.  It  should  be  noted, 
however,  that  the  best  hardening  temperature  should  be  determined 
experimentally,  whenever  possible,  for  the  particular  stock  to  be 
treated,  since  the  method  of  manufacture,  elaboration,  size  of  sec- 
tion, and  various  other  factors  all  influence  such  temperature. 

The  normal  characteristics  obtained  by  the  heat  treatment  of  1-in. 
nickel  steel  rounds  are  shown  in  the  charts  of  Figs.  165  to  169.     It 


NICKEL  STEELS 


287 


should  be  remembered  that  these  figures,  although  they  have  been 
carefully  checked  up  with  other  results  as  far  as  is  possible,  are 
experimental  figures,  and  should  be  used  with  discretion.  In  other 
words,  ordinary  commercial  heat-treatment  practice  involves  so  many 
variables,  and  especially  the  "  personal  equation,"  that  it  should 
not  be  expected  that  these  results  will  be  duplicated  in  every  instance. 


Brincll  Hardness 

Effect  of  Mass 
C.  0.25              P.  0.01 
Si.  U.09              S.  0.012 
Mn.  0.07            Ni.  3.47 

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e* 

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III 

K 

1             l/2             2            2'/i             3 

Size  in  Inches 


FIG.  171. — Effect  of  Mass  upon  the  Hardness  of  Nickel  Steel,  Water  Quenched. 

(Matthews  &  Stagg.) 


Again,  these  results  represent  the  treatment  of  l-in.  round  sections — 
so  that  while  such  results  might  be  duplicated  in  practice  with 
sections  up  to  1J  ins.  in  diameter,  further  increase  in  the  size  of 
section  will  inevitably  lower  the  physical  test  results  to  be  obtained 
under  the  same  treatment.  The  influence  of  mass  upon  the  Brinell 
hardness  is  shown  in  Figs.  170  and  171. 

On  the  other  hand,  the  normal  characteristics  for  annealed  3J  per 
cent,  nickel  steel,  as  given  in  the  diagram  in  Fig.  172,  represent  the 


288 


STEEL  AND   ITS  HEAT  TREATMENT 


average  results  which  are,  and  should  be,  obtained  in  commercial 
practice  in  the  annealing  of  the  more  common  and  larger  sections  of 
nickel  steel.  They  represent,  moreover,  the  minimum  requirements 
which  are  characteristic  of  many  existing  steel  specifications  for 
3J  per  cent,  nickel  steel,  annealed,  for  such  uses  as  engine  forgings, 


100,000 


0.35  0.40 

Per  Cent.  Carbon 


FIG.   172. — Normal  Characteristics  of  Annealed  3.5  per  cent.  Nickel  Steel. 
Large-size  Sections  of  Forgings.     Manganese  Approx.  0.6  per  cent. 


ordnance  forgings,  rolled  slabs  and  billets,  etc.,  both  for  Govern- 
ment and  commercial  uses. 

Similarly,  the  following  physical  results  for  heat-treated  work 
(quenched  and  toughened)  have  been  taken  from  various  specifica- 
tions in  order  to  show  the  minimum  results  which  may  be  expected 
in  commercial  practice.  The  manganese  requirements  are  approx- 
imately 0.50  to  0.70  per  cent.,  and  the  nickel  content  not  less  than 
3.25  per  cent. 


NICKEL  STEELS 


289 


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290 


STEEL  AND   ITS  HEAT  TREATMENT 


The  relation  which  the  maximum  size  of  section  bears  to  the 
physical  properties  of  steel  is  well  illustrated  by  the  following  speci- 
fication for  Railway  Motor  Shafts — the  steel  to  contain  3  to  4  per 
cent,  nickel: 


Max.  Dia.f 
Inches. 

Tensile  Strength, 
Lbs.  per  Sq.  In. 

Elastic  Limit, 
Lbs.  per  Sq.  In. 

Elongation,  Per 
Cent,  in  2  Ins. 

Reduction  of 
Area,  PerCent. 

3 

95,000 

65,000 

21 

50 

6 

90,000 

60,000 

22 

50 

10 

85,000 

55,000 

24 

45 

20 

80,000 

45,000 

25 

45 

over 

80,000 

45,000 

24 

40 

The  following  equations  connecting  maximum  strength,  Brin- 
ell  hardness  number  and  scleroscope  hardness  number  have  been 
computed 1  from  several  hundred  tests  made  with  nickel  steels 
of  different  carbon  content  and  heat  treated  to  bring  out  all  pos- 
sible physical  properties: 

(1)  M  =  0.71  5-32. 

(2)  M  =  3.5    S-6. 

(3)  £  =  5.0    £+48. 

M  =  maximum  strength  in  units  of  1000  Ibs.  per  sq.  in. 
B  =  the  Brinell  hardness  number. 
S  =  the  scleroscope  hardness  number. 

The  maximum  strength  corresponding  to  different  Brinell  val- 
ues as  determined  by  equation  (1)  for  these  steels  is  as  follows: 


Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

100 

39,000 

350 

216,000 

150 

74,000 

400 

252,000 

200 

110,000 

450 

287,000 

250 

145,000 

500 

323,000 

300 

181,000 

550 

358,000 

The  maximum  strength    corresponding  to  different  scleroscope 
values  as  determined  by  equation  (2),  and  the  corresponding  Brin- 


R.  R.  Abbott,  A.  S.  T.  M.,  Vol.  XV,  Part  II,  1915,  p.  43  et  seq. 


NICKEL  STEELS 


291 


ell  numbers  as  determined  by  equation  (3),  for  these  steels,  are  as 
follows : 


Scleroscope. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

20 

64,000 

148 

30 

99,000 

198 

40 

134,000 

248 

50 

169,000 

298 

60 

204,000 

348 

70 

239,000 

398 

80 

274,000 

448 

90 

309,000 

498 

100 

344,000 

548 

5    PER   CENT.    NICKEL   STEEL 

The  use  of  nickel  steel  with  the  higher  nickel  content  is  now 
largely  limited  to  case-hardening  purposes,  which  we  have  previously 
described. 

The  physical  results  obtained  from  the  treatment  of  1-in. 
sections,  containing  5  per  cent,  nickel  and  0.33  and  0.43  per  cent, 
carbon,  are  shown  in  the  charts  in  Figs.  173  and  174  respectively. 


HIGH-NICKEL   STEELS 

The  high-nickel  steels  of  25  to  35  per  cent,  nickel  are  used  prin- 
cipally for  gas-engine  valves  and  spindles,  ignition  and  boiler  tubes, 
and  for  other  similar  purposes.  These  nickel  steels  are  extremely 
tough,  dense,  have  a  high  resistance  to  shock,  a  low  coefficient  of 
expansion,  and — in  particular — are  little  subject  to  corrosion. 

Their  physical  properties  in  the  natural  condition  may  be  given 
.as  follows: 

25  to  28%  Nickel    30  to  35%  Nickel 
(0.3  to  0.5%  Carbon)         (average) 
Tensile  strength,  Ibs.  per  sq.  in. .  .    85,000  to  92,000    95,000 

Elastic  limit,  Ibs.  per  sq.  in 35,000  to  50,000    50,000 

Elongation,  per  cent,  in  2  ins.  ..  30  to  35  40 

Reduction  of  area,  per  cent ....          50  to  60  58 


These  steels  do  not  respond  to  heat  treatment,  but  may  be  an- 
nealed at  about  1450°  F.  to  facilitate  machining,  after  which  the 


292 


STEEL  AND   ITS  HEAT   TREATMENT 


(9)  jaqnmx  sssnpjwE  odoosojapg  ajoqg 

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NICKEL  STEELS 


293 


294  STEEL  AND  ITS  HEAT  TREATMENT 

physical  properties  of  the  30  to  35  per  cent,  nickel  steels  given  above 
will  average: 

Tensile  strength,  Ibs.  per  sq.  in 85,000 

Elongation,  per  cent,  in  2  ins 30 

Reduction  of  area,  per  cent 40 

Nickel  steels  with  35  to  38  per  cent,  nickel  and  0.3  to  0.5  per  cent, 
carbon  have  a  coefficient  of  expansion  which  is  less  than  any  metal 
known  and  amounts  practically  to  zero.  These  alloys  are  used  for 
various  clock,  geodetic  and  similar  instruments  for  precise  measure- 
ments. The  physical  properties  of  these  steels  are  approximately: 

Tensile  strength,  Ibs.  per  sq.  in 100,000  to  115,000 

Elastic  limit,  Ibs.  per  sq.  in 64,000  to    78,000 

Elongation,  per  cent,  in  2  ins 35  to  25 

Reduction  of  area,  per  cent 50 


CHAPTER  XII 
CHROME   STEELS 

CHROME  in  steel  has  the  characteristic  function  of  opposing  both 
the  disintegration  and  reconstitution  of  cementite.1  This  is  made 
noticeable  by  the  changes  in  the  critical  ranges  of  the  steel,  as  it 
makes  them  take  place  more  slowly,  that  is,  it  has  the  tendency  to 
raise  the  Ac  range  and  to  lower  the  Ar  range.  Chrome  steels  are 
therefore  capable  of  greater  hardness  because  rapid  cooling  is  able 
more  completely  to  prevent  the  decomposition  of  the  austenite.2 
The  greater  hardness  of  chrome  steels  is  also  due  to  the  presence  of 
double  carbides  of  chrome  and  iron  in  the  steel  in  the  hardened  or 
slightly  tempered  condition.  This  additional  mineral  hardness  is  ob- 
tained without  raising  the  brittleness  to  such  a  degree  as  does  carbon. 
The  degree  of  hardness  is  within  certain  limits  dependent  upon  the 
carbon  content,  as  chrome  alone  will  not  harden  iron.  Harbord  3 
states  that  carbonless,  or  nearly  carbonless,  chrome  steel  does  not 
harden  when  water  quenched.  Toughness  is  also  conferred  by  the 
degree  of  fineness  of  the  structure,  which  is  a  characteristic  of  chrome 
steels  (similar  to  that  of  nickel),  thus  increasing  the  tensile  strength 
and  elastic  limit  without  a  noticeable  loss  of  ductility.  Thus  we 
have  that  condition  of  "  tough-hardness  "  which  makes  chrome 
steel  so  valuable  in  parts  requiring  great  resistance  to  wear  (abrasive 
action4).  In  regard  to  corrosion,  Chappell5  states  that  "  in  neu- 
tral corroding  media  the  resistance  offered  to  corrosion  apparently 
rises  with  the  percentage  of  chromium.  This  is  particularly  the  case 
for  salt  water,  and  the  employment  of  chromium  steels  in  the  con- 
struction of  ships  would  appear  to  be  fully  justified  on  this  ground 
alone. — The  corrosion  factor  does  not  appear  to  be  a  purely  addi- 
tive quantity." 

1  Savcra,  "  Metallography  "  (trans.),  p.  315. 

2  Stoughton,  "  Metallurgy  of  Steel,"  p.  407. 

3  Harbord,  "  Metallurgy  of  Steel,"  Vol.  I,  p.  390. 

4  See  F.  Robin,  Journal.  Iron  and  Steel  Inst.,  Vol.  II,  1910. 
6  Chappell,  Journ.  Iron  and  Steel  Inst.,  1912. 

295 


296 


STEEL  AND   ITS  HEAT  TREATMENT 


The  influence  of  chrome  towards  increasing  the  brittleness  of 
steel,  especially  upon  prolonged  heating  at  high  temperatures  in  the 
case-hardening  process,  is  shown  in  the  following  results  of  tests  by 
Quillet:1 


Resistanc 

3  to  Shock. 

Treatment. 

Chrome  0.70%. 
Carbon  0.05%. 

Chrome  1.20%. 
Carbon  0.05%. 

Annealed  
Quenched  . 

32  kgms. 
22 

25  kgms. 
15 

Heated  for  4  hours  at  1830°  F. 
After  double  quenching  

5 
26 

5 
20 

i 

0.5    PER  CENT.    CHROME   STEELS 

Low-chrome  steels  find  many  valuable  uses  and  at  only  a  slightly 
increased  cost,  since  the  usual  charge  by  open-hearth  steel  manu- 
facturers for  0.5  per  cent,  chrome  steel  is  only  about  one  or  two 
dollars  a  ton  over  the  "  base  price."  One-half  of  one  per  cent, 
chrome  raises  the  critical  range  on  heating  by  about  25°  to  35°  F. 
over  that  of  the  corresponding  straight  carbon  steel.  As  a  leeway 
of  at  least  50°  F.  over  the  critical  range  is  usually  allowed  for  in 
hardening,  these  chrome  steels  may  in  general  be  hardened  at  the 
same  temperature  as  for  straight  carbon  steels  of  the  same  carbon 
content. 

0.5   CHROME,    LOW   CARBON 

For  carbons  up  to  about  0.35  per  cent,  the  addition  of  this  small 
amount  of  chrome  confers  practically  no  additional  physical  prop- 
erties other  than  those  which  might  be  obtained  by  the  use  of  a 
slightly  higher  carbon  content  in  a  straight  carbon  steel.  On  the 
other  hand,  for  case-hardening  purposes  this  small  amount  of  chrome 
confers  homogeneity,  greater  strength  and  wearing  qualities,  due  to 
the  much  finer  grain  throughout  after  the  double  quenching,  and  to 
the  presence  of  double  carbides  in  the  case.  It  should  be  remembered 
that  chrome  strengthens  the  cementitic  element  of  the  structure 
of  steel,  which  in  turn  must  depend  upon  the  amount  of  pear  lite; 
nickel,  on  the  other  hand,  influences  the  ferrite  constituent.  Al- 
though both  elements  will  tend  to  make  the  structure  much  finer, 

1 L.  Guillet,  "  Trempe,  recuit,  revenu — " 


CHROME   STEELS  297 

it  is  evident  that  while  nickel  will  have  its  greatest  effect  upon  the 
lower  carbon  steels  (those  containing  large  amounts  of  ferrite), 
chrome  will  be  of  the  most  importance  in  the  high  carbons  in  which 
there  is  considerable  cementite.  Thus  it  is  that  although  chrome 
in  small  amounts  will  be  of  little  direct  importance  in  ordinary  heat- 
treated  low-carbon  steels,  it  will  be  of  tremendous  importance  in 
any  case-hardening  operation  which  will  produce  a  high-carbon  case. 

The  addition  of  chrome  probably  increases  the  velocity  of  pene- 
tration of  the  carburization,  under  identical  conditions,  over  that 
of  the  corresponding  straight  carbon  steels.  With  greater  amounts 
of  chrome  there  is  also  the  tendency  toward  a  higher  maximum 
carbon  concentration  than  that  obtained  with  carbon  steels  similarly 
carburized.  The  tendency  to  surface  oxidation  during  carburiza- 
tion as  a  characteristic  of  chrome  steels  has  been  noted  by  several 
investigators;  methods  involving  the  use  of  mixed  cements  may  be 
used,  however,  which  will  modify  or  even  eliminate  this  action. 

As  chrome  emphasizes  the  harmful  effects  of  prolonged  heating 
(opposite  to  the  action  of  nickel),  it  is  always  necessary  to  double 
quench  the  steel;  that  is,  a  regenerative  quenching  as  well  as  the 
usual  hardening  quenching.  The  greater  surface  hardness  obtained 
by  the  use  of  chrome  steels  permits  the  use  of  oil  for  both  quenchings, 
if  desired,  and  thus  tends  to  avoid  deformation  of  the  steel. 

0.50  CHROME — 0.35  TO  0.50  CARBON 

It  has  been  the  author's  experience  that  with  a  carbon  content 
up  to  say  0.50  per  cent,  carbon,  the  addition  of  a  half  per  cent, 
chrome  will  give,  after  heat  treatment,  about  15  per  cent,  increase 
in  tensile  strength,  10  per  cent,  increase  in  elastic  limit,  with  prac- 
tically no  loss  in  ductility,  over  straight  carbon  steels  of  the  same 
carbon  content.  In  the  hardened  condition  it  gives  excellent  ser- 
vice for  wearing  surfaces,  such  as  the  jaws  of  wrenches,  small  gears, 
etc.  The  tables  on  page  298  give  test  results  obtained  from  open- 
hearth  steels. 

0.50   CHROME,    OVER    0.50    CARBON 

With  the  increase  in  carbon  the  influence  of  chrome  becomes 
even  more  marked,  due  to  the  increasing  amounts  of  double  carbide. 
The  hardness  increase  is  greater  proportionally  than  the  carbon 
increase.  For  well-bits  and  jars  in  the  hardened  condition  this  steel 
has  no  equal  among  the  low-priced  alloys  or  straight  carbon  steels. 


298 


STEEL  AND   ITS  HEAT  TREATMENT 


For  die  blocks  used  in  drop-forging  work  it  does  not  seem  quite  to 
"  hit  the  mark,"  apparently  not  having  the  requisite  toughness 
to  offset  the  brittleness,  especially  in  the  larger  sections.  With 
0.70  to  0.80  per  cent,  carbon  it  makes  an  excellent  chisel,  while  with 
0.90  to  1.00  per  cent,  carbon  and  about  0.60  per  cent  manganese  it 
gives  even  better  results  for  pneumatic  chipping  chisels  than  do 
many  varieties  of  high-speed  steel.  The  Germans  in  particular 
have  made  great  use  of  0.5  per  cent,  chrome  steels  for  tools,  such 
as  drills,  saw-blades,  knives,  razors,  files,  and  similar  tools  requir- 
ing a  keen  cutting  edge.  Further  increase  in  hardness  may  also 
be  obtained  by  the  addition  of  silicon  and  manganese.  The  mini- 
mum hardening  temperatures  for  the  higher  carbons  should  always 
be  used  to  obtain  the  maximum  effect  of  the  chrome. 


Treatment. 

C. 

Mn. 

P. 

S. 

Si.                Cr. 

1500°  F.  oil/12000   F.* 

0.36 

0.44 

0.008 

0.021 

0.05 

0.57 

1500°  F.  oil/13000  F.* 

1500°  F.  oil/13000   F  * 

As  rolled,  4"X4"  billet... 

0.40 

0.50 

0.015 

0.025 

0.55 

1460°  F.  water/1000*  .  . 

0.47 

0.60 

0.006 

0.025 

0.108 

0.51 

1460°  F.  water/1100*.  . 

1460°  F.  water/1200*.  . 

1460°  F.  water/1300*.  . 

8"  round  as  hammered  .  . 

0.47 

0  .  55 

0.015 

0.029 

0.57 

Treatment. 

Tensile 
Strength, 
Lbs.  per 
Sq.  in. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elon- 
gation, 
Per  cent, 
in  2  Ins. 

Reduc- 
tion of 
Area. 
Per  cent. 

Brinell 
Hard- 
ness. 

Sclero- 
scope 
Hard- 
ness. 

1500°  F.  oil/12000   F.* 

103,200 

76,000 

27.0 

62.0 

1500°  F.  oil/13000   F.* 

99,450 

71,240 

22.0 

51.4 

1500°  F.  oil/13000   F.* 

94,100 

67,460 

28.0 

69.0 

As  rolled,  4"x4"  billet.  . 

93,000 

72,000 

26.0 

50.0 

1460°  F.  water/1000*  .  . 

168,420 

153,720 

19.0 

52.6 

311 

44 

1460°  F.  water/1100*.  . 

143,060 

128,860 

19.0 

60.2 

277 

36 

1460°  F.  water/1200*.  . 

112,930 

120,750 

22.0 

60.5 

262 

35 

1460°  F.  water/  1300*.  . 

120,550 

109,540 

25.0 

67.6 

212 

30 

8"  round  as  hammered  .  . 

95,000 

50,000 

22.0 

50.0 

*  Tests  from  1-in.  rounds. 

Critical  range  diagrams  are  shown  in  Figs.  175  and  176. 


Results  obtained  upon  oil  quenching  from  1400°  F.  and  subse- 
quent toughening  of  1-in.  rounds  of  the  approximate  composition 
of  0.70  per  cent,  carbon,  0.60  manganese  and  0.50  chrome  are 
given  in  the  following  table;  note  especially  the  high  proportion 


CHROME  STEELS 


299 


FIG.  175. — Critical  Range  Diagram  of  Heat  2101.  Carbon,  0.47  per  cent.; 
Manganese,  0.60  per  cent.;  Phosphorus,  0.006  percent.;  Sulphur, 
0.025  per  cent.;  Silicon,  0.108  per  cent.;  Chrome,  0.51  per  cent. 


FIG.  176.— Critical  Ranges  of  Basic  Open-hearth  Steel,  Heat  8148.  Carbon, 
0.50  per  cent.;  Manganese,  0.49  per  cent.;  Phosphorus,  0.010  per 
cent.;  Sulphur,  0.026  per  cent.;  Silicon,  0.05  per  cent.;  Chrome, 
0.57  per  cent. 


300 


STEEL  AND   ITS  HEAT  TREATMENT 


of  the  elastic  limit  to  the  tensile  strength,  combined  with  good  duc- 
tility: 


Toughening 
Temperature,  °F. 

Tensile  Strength, 
Lbs.  per  Sq.  In. 

Elastic  Limit, 
Lbs.  per  Sq.  In. 

Elongation,  Per 
Cent,  in  2  Ins. 

Reduction  of 
Area,  Per  Cent. 

900 

199,500 

179,050 

6.0 

25.6 

1000 

168,900 

143,500 

12.5 

33.8 

1100 

145,700 

119,400 

15.0 

42.2 

1200 

120,000 

105,000 

17.0 

47.5 

1300 

107,100 

91,400 

22.0 

58.1 

The  critical  range  diagram  is  shown  in  Fig.  177 


FIG.  177. — Critical  Range  Diagram  of  Chrome  Carbon  Steel.  Carbon,  Approx. 
0.70  per  cent.;  Manganese,  Approx.  0.60  per  cent.;  Chrome, 
Approx.  0.50  per  cent. 

1.00    PER   CENT.    CHROME   STEELS 

Chrome  steels  with  about  1.00  per  cent,  chrome,  with  high 
carbon,  find  their  greatest  use  in  balls,  ball-races,  cones,  roller 
bearings,  crushing  machinery,  safe  steel,  tools,  and  other  parts 
requiring  a  very  hard  surface.  The  use  of  about  1  per  cent,  each 
of  carbon  and  chrome  appears  to  give  the  highest  combination 
of  "  tough-hardness  "  and  plasticity.  Such  steel  requires  care  in 
forging,  which  must  be  done  at  a  good  red  heat  and  with  powerful 
blows.  As  forged,  the  steel  is  much  too  hard  for  ordinary  ma- 
chine work  and  must  therefore  be  thoroughly  annealed. 

Annealing. — Annealing  at  the  usual  annealing  temperatures  for 
an  ordinary  length  of  time  will  not  generally  suffice,  due  to  the  slow- 


CHROME  STEELS  301 

ness  with  which  the  cementite  is  taken  into  solid  solution  by  the  aus- 
tenite,  and  which  is  well  illustrated  by  the  following  case:  A  num- 
ber of  3-in.  forged  rounds  were  annealed  at  1400°  F.  for  four  hours 
and  slow  cooled  with  the  furnace,  but  were  then  too  hard  for  machin- 
ing; they  were  reannealed  for  sixteen  hours  in  a  similar  manner, 
and  although  the  grain  was  refined,  they  could  be  sawed  only  with 
difficulty.  The  most  expeditious  method  for  annealing  this  steel 
is  to  normalize  and  then  anneal,  as  follows:  first  thoroughly  heat 
the  steel  at  a  temperature  above  the  Acm  point  for  the  "  solution  " 
of  the  cementite,  air  cool  to  a  temperature  beneath  that  of  the  Ar 
point  to  prevent  disintegration,  reheat  to  a  temperature  slightly 
over  the  Ac  1.2. 3  point  to  refine  the  grain,  and  slow  cool  in  the 
furnace  or  in  lime  to  obtain  the  maximum  degree  of  ductility  (soft- 
ness). After  such  a  treatment  the  steel  is  easily  machinable  and 
will  have  a  Brinell  hardness  of  about  130  to  170.  For  a  steel  con- 
taining 1.00  to  1.40  per  cent,  carbon,  under  0.50  per  cent,  man- 
ganese, and  about  1.00  per  cent,  chrome,  the  following  temperatures 
may  be  used  to  advantage: 

1.  Heat  to  1700°  to  1750°  F. 

2.  Air  cool  to  about  800°  F. 

3.  Heat  to  1400°  F. 

4.  Slow  cool  in  furnace  or  in  lime. 

Note. — Add  35°  to  the  temperatures  (1)  and  (3)  if  the  chrome 
is  up  to  1.50  per  cent. 

If,  however,  it  is  desired  to  anneal  the  steel  by  the  straight  anneal 
only  (i.e.,  using  but  one  temperature  and  one  heating), -this  may  be 
done  by  a  prolonged  length  of  heating,  followed  by  a  very  gradual 
and  extremely  slow  cooling.  Thus  the  3-in  rounds  previously 
referred  to  were  satisfactorily  annealed  by  heating  to  a  tem- 
perature of  about  1400°  F.,  maintaining  this  temperature  for  about 
sixty  hours,  and  then  cooling  with  extreme  slowness  through  the 
critical  range.  This  is  the  more  common  method  used  by  manu- 
facturers of  chrome  steel  for  roller  bearings  (about  1  per  cent,  car- 
bon and  1.25  to  1.50  per  cent,  chrome);  the  temperatures  vary 
between  1400°  and  1475°  F.;  the  length  of  time  varies  upon  the 
mass  of  the  charge,  and  usually  takes  several  days.  Such  steel 
in  the  full  annealed  condition  should  have  a  Brinell  hardness  of  not 
over  170. 

Hardening. — These  steels  take  on  great  hardness,  both  on  the 
surface  and  at  depth,  when  hardened,  for  which  either  water  or 
oil  may  be  used.  In  the  hardened  condition  the  Shore  scleroscope 


302 


STEEL  AND   ITS  HEAT  TREATMENT 


gives  a  hardness  figure  of  about  100.  The  critical  ranges  for  these 
steels  with  over  0.90  per  cent,  carbon  will  vary  from  1330°  to  1375° 
F.  for  0.5  per  cent,  chrome,  to  1400°  to  1450°  for  1.5  per  cent, 
chrome.  The  results  obtained  from  the  heat  treatment  of  1-in. 
rounds  of  a  0.64  per  cent,  carbon  chrome  steel  are  as  follows: 

0.64  CARBON;  0.28  MANGANESE;  0.17  SILICON;   1.04  CHROME. 


Quenched  in  Oil 
from  1600°  F. 
and  Toughened 
at  -  Deg.  F. 

Tensile 
Strength, 
Lbs.  per 
Sq.  In. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elongation, 
Per  Cent, 
in  2  Ins. 

Reduction 
of  Area, 
Per  Cent. 

Brinell 
Hardness. 

750 

227,500 

170,000 

5.0 

13.5 

477 

930 

212,000 

155,000 

8.0 

19.5 

444 

1110 

186,000 

127,500 

10.0 

22.5 

387 

2.00    PER   CENT.    CHROME   STEELS 

The  tables  on  page  303,  taken  from  the  work  of  McWilliams  and 
Barnes,1  and  rearranged,  show  the  physical  properties  of  2  per 
cent,  chrome  steels  of  ascending  carbons  as  rolled,  heat  treated  and 
annealed.  Chrome  steels  with  about  2  per  cent,  of  chrome  are 
largely  used  in  the  manufacture  of  armor-piercing  projectiles,  besides 
in  such  objects  which  require  an  extremely  hard-wearing  surface 
such  as  in  crushers,  cold  rolls,  drawing  dies,  special  files,  etc. 

HIGH-CHROME   CARBON    STEELS 

For  general  practical  purposes  and  heat-treatment  work  the 
chrome  content  is  limited  to  that  percentage  below  which  the  steel 
as  cast  will  be  pearlitic — that  is,  the  critical  temperatures  on  cool- 
ing are  all  above  atmospheric  temperatures  and  the  steel  structure 
is  composed  of  pearlite  plus  either  ferrite  or  cementite.  The  hard- 
ness increases  with  the  chrome  content,  and,  according  to  Arnold 
and  Read,  appears  to  be  independent  of  the  carbon  content,  whilst 
the  brittleness  is  far  less  than  in  carbon  steels  of  the  same  carbon 
content. 

Martensitic  Steels. — When  the  chrome  content  reaches  from 
5  to  7  per  cent.,  dependent  upon  the  carbon,  the  change  point,  Ar, 
will  fall  below  normal  temperatures  and  the  structure  will  become 
troostitic  or  martensitic.  That  is,  the  structure  will  be  comparable 
with  that  of  a  straight  carbon  steel  in  the  hardened  condition.  These 

1  "Iron  and  Steel  Inst.  Journ." 


CHROME   STEELS 


303 


2.00  CHROME — 0.20  CARBON. 


CRITICAL  RANGE  Ac3  =  1512°  F. 


Tensile 

Elastic 

Elon- 

Reduc- 

Alterna- 

Treatment. 

Strength. 

Limit, 

gation, 

tion  of 

tions 

Lbs.  per 
Sq.  In. 

Lbs.  per 
Sq.  In. 

Per  Cent, 
in  2  Ins. 

Area, 
Per  Cent. 

(Ar- 
nold's). 

1.  As  rolled  

70,400 

45,600 

30.5 

71.2 

331 

2.  1475°  F.  in  water/  750°  F. 

137,200 

134,000 

12.5 

40.6 

96 

3.  1475°  F.  in  water/10250  F. 

116,000 

110,000 

16.0 

50.7 

144 

4.  1475°  F.  in  water/13000  F. 

82,400 

64,000 

28.0 

70.2 

234 

5.  Annealed  .... 

66,000 

32,000 

40.5 

77.9 

410 

2.00  CHROME— 0.25  CARBON. 


CRITICAL  RANGE  Ac3  =  1490°  F. 


1. 

As  rolled. 

67,200 

48,800 

30.0 

68.4 

312 

2. 

1475°  F.  in  water/  750°  F. 

176,400 

156,600 

12.0 

42.5 

103 

3. 

1475°  F.  in  water/10250  F. 

144,000 

136,000 

14.5 

51.5 

99 

4. 

1475°  F.  in  water/13000  F. 

96,000 

82,000 

25.0 

68.6 

204 

5. 

Annealed  

70.000 

32.000 

39  .  5 

73  8 

437 

2.00  CHROME—  0.32  CARBON.                      CRITICAL  RANGE  Ac3  =  1445°  F. 

1. 

As  rolled. 

92,600 

60,000 

26.0 

65.4 

355 

2. 

1475°  F.  in  water/  750°  F. 

200,000 

184,000 

9.5 

37^0 

94 

3. 

1475°  F.  in  water/10250  F. 

159,200 

151,800 

15.0 

52.2 

141 

4. 

1475°  F.  in  water/13000  F. 

109,600 

94,000 

22.5 

67.2 

197 

5. 

Annealed  

60.800 

28.800 

37  0 

70  7 

482 

2.00  CHROME—  0.50  CARBON.                       CRITICAL  RANGE  Ac3  =  1432°  F. 

1. 

As  rolled. 

107,600 

64,000 

20.5 

65.8 

378 

2. 

1475°  F.  in  water/  750°  F. 

228,200 

224,000 

9.0 

30.3 

88 

3. 

1475°  F.  in  water/  1025°  F. 

179,200 

170,200 

13.0 

42.5 

111 

4. 

1475°  F.  in  water/13000  F. 

124,800 

114,000 

21.0 

61.5 

169 

5. 

Annealed  

75.200 

25.400 

28.0     ! 

55.4 

440 

2.00  CHROME  —  0.65  CARBON.                      CRITICAL  RANGE  Ac3  =  1440°  F. 

1. 

As  rolled.      .  .    .  .    . 

142,600 

116,000 

14.5 

41.1 

292 

2. 

1475°  F.  in  water/  750°  F. 

3. 

1475°-F.  in  water/10250  F. 

193,000 

188,200 

10.0 

32.4 

74 

4. 

1475°  F.  in  water/13000  F. 

125,200 

113,600 

21.0 

55.6 

133 

5. 

Annealed 

97.800 

64.000 

21   5 

62  2 

214- 

2.00  CHROME  —  0.85  CARBON.                      CRITICAL  RANGE  Ac3  =  1430°  F. 

1. 

As  rolled  

151,800 

104,000 

10.0 

18.3 

178 

2. 

1475°  F.  in  water/  750°  F. 

3. 

1475°  F.  in  water/10250  F. 

191,400 

185,000 

8.5 

28.2 

65 

4. 

1475°  F.  in  water/13000  F. 

126,000 

115,000 

20.0 

51.7 

155 

5. 

Annealed        .                      .  . 

80,200 

37,600 

32.0 

63.5 

316 

304 


STEEL  AND   ITS  HEAT  TREATMENT 


steels  have  high  tensile  strength  and  elastic  limit,  low  ductility, 
great  hardness  and  medium  brittleness.  Heat  treatment  has  little 
or  no  influence,  except,  perhaps,  to  refine  the  grain.  On  account  of 
their  physical  characteristics  these  steels  are  but  little  used,  except 
as  applied  to  special  tool  steel.  As  the  chrome  content  is  again 
increased  to  about  12  to  15  per  cent.,  intensely  white  grains  of  the 
double  carbide  of  chrome  and  iron  form  within  the  martensite,  and 


18 


Double  Carbide 


Martensite 


Mart. 


Pearlite 


Double 
Carbide 


1.65 


2.20 


FIG.  178.  —  Microscopic  Constituents  of  Chrome  Carbon  Steels. 


gradually  occupy  the  whole  field  with  further  increase  of  chrome. 
These  structural  changes  for  varying  percentages  of  chrome,  and  with 
0.2  and  0.8  per  cent,  carbon  respectively,  are  given  by  Guillet  as 
follows: 


Structure  With  0.2  Carbon 

Pearlitic 0  to    7  per  cent.  Cr. 

Troostitic 7  to    8  per  cent.  Cr. 

Martensitic 8  to  13  per  cent.  Cr. 

Martensiteplusdouble  carbide  13  to  20  per  cent.  Cr.  j  lg  and  oyer        cent 
Double  carbide ...          over  20  per  cent.  Cr.  J 


With  0.8  Carbon 
0  to    5  per  cent.  Cr. 

5  to  18  per  cent.  Cr. 


per  cent. 
These  changes  are  shown  graphically  in  Fig.  178. 


CHROME  STEELS 


305 


The  effect  of  annealing  and  heat  treatment  upon  high-chrome 
carbon  steels,  with  approximately  0.4  per  cent,  carbon  is  given  in 
the  following  table:1 

HEAT  TREATMENT  OF  HIGH-CHROME  STEELS,  0.4  CARBON 


Chrome, 
Per  Cent 

Treatment. 

Tensile 
Strength, 
Lbs.  per 
Sq.  In. 

Elastic 
Limit 
Lbs.  per 
Sq.  In. 

Elon- 
gation 
Per  cent, 
in  2  Ins. 

Reduction 
of  Area, 
Per  Cent. 

Annealed  

53,760 

39,872 

24 

24.0 

5 
10 
15 

Hardened  &  tempered 
Annealed  
Hardened  &  tempered 
Annealed  
Hardened  &  tempered 
Annealed 

123,648 
94,080 
121,632 
101,472 
130,144 
80,864 

109,312 
51,296 
94,976 
56,896 
109,312 
47,488 

12 
21.5 
12 
18.5 
11.5 
21  5 

37.0 
44.0 
53.6 
50.0 
54.6 
46  5 

20 
25 

Hardened  &  tempered 
Annealed  
Hardened 

90,272 
94,526 
90,496 

61,824 
66,752 
61,824 

19.5 
18 
20 

51.5 
62.1 
50  0 

Annealed  . 

93,184 

71,232 

19 

62  0 

30 

Hardened 

87360 

64518 

19 

65  0 

Further  data  on  high-chrome  steels  may  be  obtained  from  the 
researches  of  Guillet,2  Portevin,3  Arnold  and  Read,4  Becker,5 
Mars,6  and  others. 

1  J.  Holtzer  &  Cie.,  Loire,  France,  from  Harbord's  "  Metallurgy,"  I,  391. 

2  Guillet,  "  Les  Aciers  Speciaux." 

3  A.  Portevin,  "  Revue  de   Metallurgie,"   1909,   No.   12,  p.   1264,   "  Metal- 
lurgie,"  1910,  Heft  6,  s.  177. 

4  Arnold  and  Read,  "  Iron  and  Steel  Inst.  Journ." 
*  O.  M.  Becker,  "  High-speed  Steel." 

6  Mars,  "  Spezialstahle."  1912. 


CHAPTER  XIII 
CHROME    NICKEL   STEELS 

Chrome  Nickel  vs.  Chrome  Vanadium  Steels. — Chrome  nickel 
steels,  as  a  type  composition,  probably  represent  the  best  all-around 
alloy  steels  in  commercial  use  for  general  purposes.  By  this  it  is 
not  to  be  inferred  that  chrome  nickel  should  always  be  used  in  pref- 
erence to  other  alloys;  as  a  matter  of  fact,  each  type  is  more  or  less 
peculiarly  adapted  to  work  of  a  distinctive  nature.  On  the  other 
hand,  chrome  nickel  steel  of  suitable  composition  will  satisfy  nearly 
every  conditipn  for  structural  and  similar  purposes.  Much  has  been 
said  and  done  with  chrome  vanadium  steels,  and  while  the  latter 
undoubtedly  do  fill  a  long-felt  want  along  certain  lines,  it  should  not 
be  said  that  chrome  vanadium  steels  are  superior  to  chrome  nickel 
steels.  In  fact,  with  a  few  exceptions,  chrome  nickel  steels  of  suitable 
composition  will  generally  measure  up  to  any  standards  set  by  the 
ordinary  vanadium  alloys  and  at  equal  or  at  even  less  cost.  Neither 
chrome  vanadium,  nor  chrome  nickel,  nor  any  one  type  of  steel  is  a 
general  prescription  for  the  every  ill  of  the  steel  user:  each  steel  has  its 
distinctive  characteristics  and  applications.  And  notwithstanding 
the  mass  of  advertising  "  literature  "  to  the  contrary,  it  would  also 
be  decidedly  improper  to  state,  as  a  general  rule,  that  either  is 
superior  to  the  other. 

Influence  of  Chrome  and  Nickel. — Chrome  nickel  steels  of  suit- 
able composition  appear  to  have  the  beneficial  effects  of  both  the 
chrome  and  nickel,  but  without  the  disadvantages  which  are  inherent 
in  the  use  of  either  one  separately.  Moreover,  the  presence  of  both 
chrome  and  nickel  seems  to  intensify  certain  physical  characteristics. 
To  the  increased  ductility  and  toughness  conferred  by  nickel  on  the 
ferrite  there  is  added  the  mineral  hardness  given  to  the  cement ite 
and  pear  lite  by  the  chrome,  but  with  a  greater  resultant  effect. 
Again,  while  the  addition  of  nickel  alone  serves  to  diminish  the 
susceptibility  to  brittleness  in  the  steel  upon  prolonged  heating  or 
sudden  cooling — in  comparison  with  the  corresponding  straight 
carbon  steels — and,  on  the  other  hand,  the  presence  of  chrome 

306 


CHROME  NICKEL  STEELS  307 

alone  tends  to  the  opposite  effect,  a  suitable  combination  of  the  two 
alloying  elements  tends  to  neutralize  the  harmful  effects  and  also  to 
magnify  the  good  points.  This  is  not  only  brought  out  in  the 
static  strength  and  ductility,  but  also  in  the  dynamic  strength  or 
fatigue  resistance. 

Statements  have  frequently  appeared  in  print  to  the  effect  that 
nickel  "  poisons  "  the  steel  dynamically;  that  chrome  has  little 
influence  one  way  or  the  other  upon  the  fatigue  resistance;  and  that 
chrome  nickel  steels  are  inferior  along  these  lines  to  certain  other 
specific  alloy  steels.  In  considering  these  broad  statements  there 
are  three  things  in  particular  which  should  be  noted.  Firstly,  that 
in  the  present  state  of  the  art  of  dynamic  strength  testing,  the  re- 
sults so  obtained  are  often  widely  divergent  for  the  same  steel,  not 
to  mention  any  comparison  of  results  upon  different  steels  of  dis- 
similar type.  Secondly,  even  assuming  that  concordant  and  strictly 
comparative  results  could  be  thus  obtained  by  means  of  the  testing 
machines  now  in  use,  the  majority  of  the  experimental  results  pub 
forth  to  prove  the  general  inferiority  (dynamically)  of  chrome 
nickel  steels  in  relation  to  certain  other  types  (e.g.,  chrome  vanadium) 
are  oftentimes  not  really  comparative  at  all,  since  the  two  distinct- 
ive types  of  steel  have  been  heat  treated  alike.  That  is,  while  it 
may  be  perfectly  good  practice  to  quench  a  chrome  vanadium  steel 
from  say  1650°  F.,  it  might  be  distinctively  poor  practice  to  quench 
a  chrome  nickel  steel  from  the  same  temperature.  And  yet  many 
"  comparative  "  results  have  been  obtained  in  just  such  a  manner, 
to  the  detriment  of  either  one  steel  or  the  other.  Rather,  then, 
should  each  steel  be  treated  in  that  impersonal  and  strictly  scientific 
manner  which  will  tend  to  bring  out  the  maximum  qualities  of  each; 
and  then  should  the  tests  be  made  upon  the  same  machine  under  like 
conditions.  Thirdly,  whatever  may  be  the  influence  of  chrome 
or  nickel  alone  upon  the  dynamic  strength  of  steel,  it  has  been  re- 
peatedly demonstrated  that  the  proper  combination  of  the  two 
alloys  undoubtedly  produces  a  type  of  metal  with  vastly  improved 
capacity  for  resistance  to  fatigue. 

Commercial  Ratio  of  Chrome  and  Nickel  Content. — From  the 
author's  experience  in  both  the  manufacture  and  use  of  chrome 
nickel  steels  it  would  appear  that  there  is  some  ratio  existing  between 
the  proportion  of  the  chrome  and  nickel  which  will  give  the  most 
efficient  combination  of  physical  characteristics.  In  other  words, 
by  combining  the  chrome  and  nickel  in  some  such  ratio,  the  less 
susceptibility  to  brittleness  upon  prolonged  heating  which  is  char- 


308  STEEL  AND   ITS  HEAT  TREATMENT 

acteristic  of  nickel  additions  will  modify  the  greater  susceptibility 
to  brittleness  which  is  given  by  chrome  alone,  giving  a  stronger 
and  better  steel  than  may  be  obtained  when  this  ratio  is  not  ob- 
served. Again,  it  will  be  observed  that  if  the  chrome  content  greatly 
exceeds  a  certain  proportion  in  respect  to  the  nickel,  the  steel  will  be 
more  difficult  to  heat  treat  successfully,  the  temperature  limits  are 
more  narrow,  and  the  possibility  of  poor  results  is  greatly  increased. 
This  best  ratio  is  probably  about  2J  parts  of  nickel  to  about  1  part 
of  chrome.  Thus  we  have  the  principal  standard  types  of  3.5  nickel 


FIG.  179.— Protective  Deck  Steel.     (Bullens.) 

and  1.5  chrome,  1.5  nickel  and  0.6  chrome,  and  various  intermediate 
types. 

Carburization. — The  carburization  of  chrome  nickel  steels  does 
not  differ  in  principle  from  that  previously  described.  These  steels 
generally  carburize  more  rapidly  and  better  than  straight  carbon 
steels,  and,  in  particular,  give  the  characteristic  gradual  cemented 
zone  which  should  always  be  aimed  for.  The  presence  of  suitable 
proportions  of  chrome  and  nickel,  as  previously  mentioned,  also 
gives  that  low  brittleness  of  core  which  is  so  desirable;  this  fact 
even  permits  the  use  of  steels  up  to  some  0.3  per  cent,  carbon  without 


CHROME   NICKEL  STEELS  309 

great  danger.  The  use  of  chrome  nickel  steel  in  case-hardening 
work  covers  a  wide  range — from  small  gears  subject  to  great  shock 
and  wear  to  the  heaviest  grades  of  armor  plate. 

Heat  Treatment.— The  heat  treatment  of  these  steels  does  not 
present  any  new  problems.  In  the  main  the  discussion  under  the 
chapters  on  Carbon  Steels  and  Nickel  Steels  will  apply  equally  well 
to  chrome  nickel  steels.  Similarly  to  nickel  steels,  these  steels  are 
less  susceptible  to  the  deleterious  influence  of  high  temperatures, 
and  which  will  be  subsequently  mentioned.  Suitable  heat  treatment 
will  develop  a  very  fine  micro-structure,^  is  shown  in  Fig.  179, 
representing  the  structure  of  specially  treated  chrome  nickel  steel 
used  for  protective  deck  plate  on  battleships;  4he  physical  proper- 
ies  on  this  particular  steel  were:  A 

Tensile  strength,  Ibs.  per  sq.  in 132,000 

Elastic  limit,  Ibs.  per  sq.  in 116,700 

Elongation,  per  cent,  in  2  ins 23 

Reduction  of  area,  per  cent 64 

Proper  annealing  will  likewise  develop  a  good  micro-structure 
in  the  steel,  as  is  shown  in  Fig.  40.  The  critical  ranges  of  chrome 
nickel  steels  are  somewhat  lower  than  those  of  the  corresponding 
straight  carbon  steels,  so  that  lower  temperatures  may  be  used  for 
quenching. 

In  general,  the  best  treatments  which  can  be  given  to  these  alloy 
steels  after  forging  are  as  follows: 


a.  Quench  in  oil  from  about  175°  to  200°  F.  over  the  critical 

range. 
6.  Quench  in  oil  from  about  50°  over  the  critical  range. 

c.  Anneal  at  about  75°  under  the  critical  range  (also  see  II). 

Machine. 

d.  Quench  in  the   proper  medium  from  about  50°  over  the 

range. 

e.  Draw  the  temper  to  suit  the  work  in  hand. 

II 

For  shafts  and  other  structural  parts  in  which  the  desired  physical 
properties  may  be  obtained  by  a  drawing  temperature  of  about  600° 


310  STEEL  AND   ITS  HEAT  TREATMENT 

F.  or  over,  and  which  will  leave  the  steel  in  a  machinable  condition, 
Treatment  I  may  be  modified  at  (c)  as  thus  noted,  and  no  further 
treatment  will  be  required.  But  if  the  drawing  temperature  must 
be  much  lower,  as  for  gears,  the  full  treatment  as  in  (I)  is  advisable. 

a.  Quench  in  oil  from  about  175°  to  200°  F.  over  the  critical 

range. 

b.  Quench  in  oil  from  about  50°  over  the  critical  range. 

c.  Draw  at  900°  or  more,  as  the  work  may  require.     Machine. 


Ill 

The  full  treatment  'as  given  under  (I)  may  be  modified,  if 
desired,  to  the  following,  for  parts  to  be  drawn  below  900°  or 
1000°  F.: 

a.  Quench  in  oil  from  about  175°  to   200°  over  the  critical 

range. 

b.  Reheat  to  about  25°  to  50°  F.  over  the  critical  range  and 

cool  slowly.     Machine. 

c.  Quench  in  oil  from  about  50°  over  the  critical  range. 

d.  Draw  to  the  temperature  required  by  the  work. 


LOW    CHROME  NICKEL   STEELS 

The  low  chrome  nickel  steels,  containing  approximately  0.5 
per  cent,  chrome  and  1.5  per  cent,  nickel,  are  the  most  used  of  all 
the  chrome  nickel  alloys.  After  forging  or  rolling,  this  grade  of 
synthetic  steel  may  be  heat  treated  to  develop  physical  character- 
istics nearly  equivalent  to  a  3.5  per  cent,  nickel  steel  of  similar  car- 
bon content.  It  does  not  have  the  objectionable  tendency  to 
laminate  which  may  characterize  the  latter  steel,  and  on  account 
of  the  less  cost  of  alloys,  this  chrome  nickel  steel  is  sold  at  a  price 
considerably  lower  than  that  of  3.5  per  cent,  nickel  steel.  This 
grade  of  chrome  nickel  steel  forges  well  and  machines  easily,  does 
not  require  the  more  narrow  temperature  limits  in  heat  treatment  as 
do  some  steels  containing  a  larger,  although  not  as  well  proportioned, 
amount  of  chrome  and  nickel. 

The  physical  tests  obtained  from  heat-treated  steel  of  1-in. 
sections,  containing  approximately  0.5  per  cent,  chrome  and  1.5 
per  cent,  nickel,  are  given  in  the  charts  in  Figs.  180  to  183. 


CHROME  NICKEL  STEELS 


311 


312 


STEEL  AND   ITS  HEAT  TREATMENT 


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CHROME  NICKEL  STEELS 


313 


314 


STEEL  AND  ITS  HEAT  TREATMENT 


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Chemical  Analysis:  Critical  Ranges:  Size  of  Section: 
C.  0.545  Ao  T300°-1320°  1  inch  round 
Mn.  0.50  Ar  U80° 

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PH 


CHROME  NICKEL  STEELS 


315 


Small  automobile  forgings  specifying  0.20  to  0.27  per  cent, 
carbon  (with  the  above  amount  of  chrome  and  nickel)  may  readi]y 
be  heat  treated  to  fulfill  the  requirements  of: 

Tensile  strength,  Ibs.  per  sq.  in 100,000 

Elastic  limit,  Ibs.  per  sq.  in 85,000 

Elongation,  per  cent,  in  2  ins 16 

The  critical  range  of  steels  of  this  analysis  is  about  1425-1450°  F.; 
a  quenching  temperature  of  1475°  to  1500°  will  generally  give  the 
best  results  in  small  sections  which  only  require  a  single  quenching. 

An  interesting  comparison  between  this  grade  of  chrome  nickel 
steel  and  3J  per  cent,  nickel  steel  with  the  same  carbon  content  is 
shown  by  the  following  requirements  for  automobile  axles  of  a  diam- 
eter of  If  ins.  as  specified  by  one  manufacturer; 

SPECIFICATION  FOR  HEAT-TREATED  AUTOMOBILE  AXLES,  1^-m-     DIAM. 


Chrome  Nickel 
Steel. 

Nickel  Steel. 

Carbon  

0.30  to  0.40 

0.30  to  0.40 

Manganese  

0.50  to  0.70 

0.50  to  0.70 

Phos.  and  sul.. 

under  0  .  04 

under  0.04 

Silicon  

0.20 

0.20 

Nickel 

1.50 

3.50 

Chrome 

0.50 

Tensile  strength,  Ibs.  per  sq.  in  
Elastic  limit,  Ibs.  per  sq.  in. 

120,000 
110,000 

135,000 
120,000 

Elongation  per  cent  in  2  ins 

16 

16.5 

Reduction  of  Area,  per  cent  
Bend  test,  flat  around. 

45 

180° 

50 

180° 

The  following  results  were  obtained  from  test  pieces  taken  from 
full-size  forgings  of  approximately  8  to  10  ins.  in  diameter,  and 
having  an  analysis  of  carbon  0.38  per  cent.,  manganese  0.55  per 
cent.,  chrome  0.31  per  cent.,  and  nickel  1.20  per  cent.  Many 
interesting  points  were  noticed  in  the  treatment  of  this  grade  of 
steel  approximating  the  analysis  given,  and  especially  the  seeming 
contradiction  of  the  annealing  and  hardening  temperatures.  The 
critical  range  of  this  steel  is  approximately  1340°  to  1360°  F.  Forg- 
ings annealed  at  temperatures  slightly  above  these  show  perfect 
annealing.  If  the  annealing  temperature  should  be  raised,  it  is  at 
the  expense  of  ductility,  and  the  fracture  becomes  coarsely  crystalline 


316 


STEEL   AND   ITS   HEAT  TREATMENT 


and   shows    "fire."     The    physical    properties   thus    obtained    are 
shown  in  the  following  table : 


Treatment. 

Tensile 
Strength, 
Lbs.  per 
Sq.  In. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elonga- 
tion 
Per  Cent. 
in  2  Ins. 

Reduction 
of  Area, 
Per  Cent. 

As  forged                            .  . 

86,500 

45,000 

21 

34.6 

Annealed  at  1360°-1380° 

75,250 

35,500 

33 

57.9 

74,300 

35,000 

30 

50.1 

Annealed  at  1550°-1575° 

77,750 

42,000 

17 

26.0 

Unless  previously  normalized  (as  by  air  cooling  from  a  temper- 
ature such  as  used  in  the  high-temperature  anneal  above) ,  or  double 
quenched  as  outlined  in  the  treatments  previously  given,  the  harden- 
ing of  sections  of  say  3  ins.  diameter  or  more  demands  the  use  of 
a  temperature  some  200°  over  the  critical  range  to  bring  out 
the  full  effects  of  this  combination  of  chrome  and  nickel.  This 
is  true  of  both  oil  and  water  quenching,  but  more  noticeably  so 
in  the  case  of  oil  baths.  Parenthetically,  it  is  interesting  to  compare 
such  treatment  with  that  which  has  been  previously  described  under 
nickel  steels;  the  technical  reasons  will  then  be  clear.  It  is  the 
author's  experience,  as  well  as  that  of  numerous  others,  that  a 
hardening  temperature  of  1550°  to  1580°  F.  is  required  if  the  steel 
has  not  been  previously  treated;  take  for  example  the  following 
tests  on  an  8-in  round  bar: 


Quenched  in 

Drawn 

Tensile 

Elastic 

Elongation, 

Reduction 

Oil  from 

at  Deg. 

Strength, 

Limit, 

Per  Cent. 

of  Area, 

Deg.  F. 

F. 

Lbs.  per  Sq.  In. 

Lbs.  per  Sq.  In. 

in  2  Ins. 

Per  Cent. 

1450 

900 

79,750 

46,750 

28.0 

58.9 

1500 

900 

88,300 

53,000 

23.5 

55.4 

1580 

1050 

99,000 

71,500 

23.5 

61.9 

1580 

1050 

92,200 

63,600 

27.0 

62.4 

1580 

1050            95,880 

70,700 

24.0 

62.0 

It  will  be  noticed  that  the  quenching  heat  of  1580°  F.  not  only  gives 
higher  tensile  strength,  elastic  limit  and  ductility,  but  also  permits 
of  a  drawing  temperature  some  150°  higher.  Microscopically  the 
structure  obtained  by  the  high-quenching  temperature  is  excellent, 
as  is  shown  in  Fig.  184.  The  structure  of  the  same  piece  after 
forging,  and  before  treatment,  is  shown  in  Fig.  185. 

Steel  with  approximately  0.50  per  cent,  chrome,  1.50  per  cent, 
nickel  and  about  0.40  carbon  may  be  readily  heat  treated  to  fulfill 


CHROME  NICKEL  STEELS 


317 


FIG.  184.— Chrome  Nickel  Steel  Axled,  Oil  Quenched  from  1580°  F.,  Drawn  at 
1050°  F.     X100.     (Bullens.) 


FIG.  185.— Chrome  Nickel  Steel  Axle  as  Forged.     X100.     (Bullens.) 


318 


STEEL  AND  ITS  HEAT  TREATMENT 


the  specification,  in  large  sections  up  to  12  ins.  diameter,  and  with 
proportionally  higher  tensile  results  in  small  sections,  of: 

Tensile  strength,  Ibs.  per  sq.  in 90,000 

Elastic  limit,  Ibs.  per  sq.  in 60,000 

Elongation,  per  cent,  in  2  ins 22 

Reduction  of  area,  per  cent 50 

The  following  equations  connecting  maximum  strength,  Brin- 
ell  hardness  number  and  scleroscope  hardness  number  have  been 
computed l  from  several  hundred  tests  made  with  low  chrome 
nickel  steel  (1.5  per  cent,  nickel  and  0.5  per  cent,  chrome)  of  different 
carbon  content  and  heat  treated  to  bring  out  all  possible  physical 
properties: 

(1)  M  =  0.68  B -22. 

(2)  M  =  3.7    S-l. 

(3)  B  =  5A    S+33. 

M= maximum  strength  in  units  of  1000  Ibs.  per  sq.  in. 
#  =  the  Brinell  hardness  number. 
S=the  scleroscope  hardness  number. 

The  maximum  strength  corresponding  to  different  Brinell  val- 
ues as  determined  by  equation  (1)  for  these  steels  is  as  follows: 


Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

100 

46,000 

350 

216,000 

150 

80,000 

400 

250,000 

200 

114,000 

450 

284,000 

250 

148,000 

500 

318,000 

300 

182,000 

550 

352,000 

The  maximum  strength  corresponding  to  different  scleroscope 
values  as  determined  by  equation  (2),  and  the  corresponding  Brin- 
ell numbers  as  determined  by  equation  (3),  for  these  steels,  are  as 
follows: 


1R.  R.  Abbott,  A.  S.  T,  M.,  Vol.  XV,  Part  II,  1915,  p.  43  et  seq. 


CHROME  NICKEL  STEELS 


319 


Scleroscope. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

20 

73,000 

141 

30 

110,000 

195 

40 

147,000 

249 

50 

184,000 

303 

60 

221,000 

357 

70 

258,000 

411 

80 

295,000 

465 

90 

332,000 

519 

100 

369,000 

573 

HIGH    CHROME    NICKEL    STEELS 

Chrome  nickel  steels  containing  approximately  3.5  per  cent, 
nickel  and  1.5  per  cent,  chrome  comprise  a  type  of  steel  with  dis- 
tinctive physical  characteristics,  but  which  obviously  are  not  shown 
by  the  results  of  ordinary  pull  test  values  when  taken  in  comparison 
with  the  low  chrome  nickel  steels.  The  following  figures,  giving  the 
ordinary  physical  properties,  illustrate  the  latter  point.  Dependent 
upon  the  section,  treatment,  and  carbon  content  (0.2  to  0.5  per 
cent.),  they  may  be  given  as  follows: 


Composition. 

Tensile  Strength. 

Elastic  Limit. 

Elongation. 

Reduction  of  Area. 

3.5  Nickel 
1  .  5  Chrome 

85,000  to 
275,000 

55,000  to 
265,000 

26  to  10 

65  to  35 

1  .  5  Nickel 
0  .  5  Chrome 

80,000  to 
264,000 

56,000  to 
240,000 

30  to  8 

70  to  27.  5 

It  is  evident,  since  the  above  results  show  but  little  difference, 
that  the  superiority  of  the  high  chrome  nickel  steel  does  not  appear 
in  the  static  properties.  On  the  other  hand,  there  is  a  tremendous 
difference  between  the  two  types  (in  favor  of  the  higher  alloy)  in 
the  dynamic  and  endurance  strength,  such  as  freedom  from  brittle- 
ness  and  resistance  to  shock.  This  is  illustrated  by  certain  specific 
uses,  as  examples,  to  which  these  steels  are  put  and  which  demand 
the  highest  attainable  combination  of  dynamic  strength,  resistance 
to  shock,  and  high  static  strength.  Thus  with  about  0.2  to  0.3 
per  cent,  carbon  these  steels  are  used  in  protective  deck  plate, 
requiring  that  peculiar  combination  of  properties  which  comprise 
ballistic  strength;  with  a  slightly  higher  carbon  content,  and  cer- 
tain other  modifications,  we  have  a  typical  Krupp  armor  plate;  and 


320 


STEEL  AND   ITS  HEAT  TREATMENT 


with  0.45  to  0.50  per  cent,  carbon  these  steels  are  used  in  high-duty 
gears,  and  in  which  it  is  possible  to  hammer  one  tooth  against  its 
neighbor  without  breaking  it  off. 

Or,  as  it  has  been  expressed  in  e very-day  terms,  the  effect  of  the 
larger  amounts  of  alloys  in  suitable  combination  is  like  a  comparison 
between  a  trained  athlete  and  the  amateur.  Each  man  may  be 
able  to  lift  a  maximum  weight  of  say  200  Ibs.  But  when  it  comes 
to  repeating  that  same  feat  a  number  of  times  in  succession,  the 
trained  man,  with  his  developed  powers  of  endurance,  will  win 
every  time.  And  thus  it  is  with  the  high  alloy  steel. 

Typical  results  for  a  steel  of  this  type  are  given  in  the  chart  in 
Fig.  186. 

The  following  equations  connecting  maximum  strength,  Brin- 
ell  hardness  number  and  scleroscope  hardness  number  have  been 
computed  1  from  several  hundred  tests  made  with  high  chrome- 
nickel  steel  (3.5  per  cent,  nickel  and  1  per  cent,  chrome)  of  different 
carbon  content  and  heat  treated  to  bring  out  all  possible  physical 
properties : 

(1)  M  =  0.7lB-33. 
"  (2)  M  =  3.7    S-3 

(3)    £=4.8    ,S+58. 

M  =  maximum  strength  in  units  of  1000  Ibs.  per  sq.  in. 
#  =  the  Brinell  hardness  number. 
$  =  the  scleroscope  hardness  number. 

The  maximum  strength  corresponding  to  different  Brinell  val- 
ues as  determined  by  equation  (1)  for  these  steels  is  as  follows: 


Brinell  . 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

100 

38,000 

350 

215,000 

150 

73,000 

400 

251,000 

200 

109,000 

450 

286,000 

250 

144,000 

500 

322,000 

300 

180,000 

550 

357,000 

The  maximum  strength    corresponding  to  different  scleroscope 
values  as  determined  by  equation  (2),  and  the  corresponding  Brin- 


»R.  R.  Abbott,  A.  S.  T.  M.,  Vol.  XV,  Part  II,  1915,  p.  43  et  seq. 


CHROME   NICKEL  STEELS 


321 


wwiipjirn  IpuiJQ 

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322 


STEEL  AND  ITS  HEAT  TREATMENT 


ell  numbers  as  determined  by  equation  (3),  for  these  steels,  are  as 
follows: 


Scleroscope. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

20 

71,000 

154 

30 

108,000 

202 

40 

145,000 

250 

50 

182,000 

298 

60 

219,000 

346 

70 

256,000 

394 

80 

293,000 

442 

90 

330,000 

490 

100 

367,000 

538 

INTERMEDIATE   TYPES    OF   CHROME   NICKEL   STEELS 

Between  the  high  and  low  composition  types  of  chrome  nickel 
steels  previously  given  there  are  a  great  variety  of  combinations  of 
the  two  alloying  elements.  Thus,  in  the  chart  in  Fig.  187,  we  have 
the  results  obtained  from  the  heat  treatment  of  1-in.  rounds  with 
1.0  per  cent,  chrome  and  1.75  per  cent,  nickel.  Other  results, 
from  similar  compositions,  taken  from  representative  practice  in  the 
automobile  world  are  given  as  follows : 

CARBON  0.26  TO  0.35. 


Treatment. 

Tensile 
Strength, 
Lbs.  per  Sq.  In. 

Elastic 
Limit, 
Lbs.  per  Sq.  In. 

Elongation, 
Per  Cent, 
in  2  Ins. 

Reduction 
of  Area, 
Per  Cent. 

Brinell 
Hardness. 

Hardened.  .  .  . 
Toughened.  .  . 
Untreated  .... 

197,000 
110,000 
106,000 

135,000 
90,000 
70,000 

9 
25 

18 

37 
55 
45 

460  to  480 
235  to  250 

CARBON  0.46  TO  0.55. 


Tempered  .... 

305,000 

265,000 

5 

16 

480  to  525 

Toughened.  .  . 

130,000 

114,000 

20 

60 

300  to  335 

Annealed  

95,000 

68,000 

26 

50 

180  to  200 

The  effect  of  mass  upon  the  latter  type  of  chrome  nickel  steel  is 
shown  in  Fig.  188. 

The  chart  in  Fig.  189  gives  the  results  of  tests  upon  steel  con- 
taining 0.75  per  cent,  chrome  and  3.0  per  cent,  nickel,  while  Fig.  190 
illustrates  a  characteristic  French  steel  containing  0.50  per  cent, 
chrome  and  2.50  per  cent,  nickel. 


CHROME  NICKEL  STEELS 


323 


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Chemical  Analysis:  Critical  R^ges:  Size  of  Section:  Treatment: 
C.  0.45  Acl.2.3  1390°  1  Inch  round  Previously  double 
Mn.  0.45  Ar3.2.1  1250°  quenched  iu  Oil 

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324 


STEEL  AND  ITS  HEAT  TREATMENT 


Steels  with  0.60  per  cent,  chrome,  3.5  per  cent,  nickel,  and 
0.2  to  0.4  per  cent,  carbon,  in  medium-size  forgings,  may  readily 
be  treated  to  give  a  minimum  of: 

Tensile  strength,  Ibs.  per  sq.  in 120,000 

Elastic  limit,  Ibs.  per  sq.  in 105,000 

Elongation,  per  cent,  in  2  ins 20 

Illustrative  of  the  relation  of  drawing  temperatures  to  the  carbon 


Effect  of.  M 

C.  0.50  P.  0.01 

Si.  0.16  S.  0.011 

Mn.  0.41  Cr. 

Ni.  2.02 


200 


1  I1/*  2 

Size  in  Inches 


FIG.  188. — Effect  of  Mass  upon  the  Hardness.      (Matthews  &  Stagg.) 

content  for  steels  of  this  composition  and  with  the  same  size  of 
section,1  to  meet  the  above  specification,  the  following  may  be  of 
interest: 


Per  Cent.  Carbon. 

Drawing  Temperature. 

0.24 
0.27 
0.36 

1150°  F. 
1200°  F. 
1240°  F. 

1  Protective  Deck  Plate. 


CHROME  NICKEL  STEELS 


325 


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326 


STEEL  AND  ITS  HEAT  TREATMENT 


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Chemical  Analysts:  Critical  Range:  Size  of  Sett  ioa»  Treatment! 
C.  0.33  Ac  1120°  Itoltfincli  Quenched 
Wn.  0.42  Ar  1230°  in  water 
Si.  0.14  fromlDCOf 
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CHROME   NICKEL  STEELS 


327 


SPECIAL   CHROME    NICKEL    STEELS 


The  following  tests  by  Revillon  on  various  combinations  of  car- 
bon, chrome  and  nickel  in  chrome  nickel  steels  will  be  of  interest: 


ANALYSES 


No. 

Carbon. 

Man- 
ganese. 

Phos- 
phorus. 

Sulphur. 

Silicon. 

Chrome. 

Nickel. 

1 

.22 

.54 

.009 

.044 

.36 

.35 

2.19 

2 

.25 

.25 

.027 

.043 

.08 

.48 

2.75 

3 

.425 

.27 

.006 

.042 

.20 

1.20 

2.86 

4 

.17 

.53 

.006 

.053 

.16 

.18 

3.47 

5 

.105 

.43 

.014 

.030 

.11 

.85 

4.38 

6 

.86 

.23 

.014 

.030 

.11 

.86 

.88 

7 

.25 

.52 

.006 

.053 

.   17 

1.28 

3.82 

8 

.31 

.70 

.014 

.021 

.17 

1.48 

2.75 

9 

.42 

.22 

.013 

.057 

.11 

.31 

4.09 

10 

.45 

.28 

.014 

.030 

.11 

.58 

2.25 

11 

.52 

.27 

.006 

.030 

.39 

.43 

2.80 

12 

.77 

.32 

.014 

.030 

.11 

.19 

1.13 

13 

.10 

.35 

.003 

.035 

.31 

1.75 

5.36 

14 

.265 

.24 

.014 

.030 

1.27 

2.33 

4.40 

15 

.27 

.39 

.014 

.030 

.11 

.85 

4.90 

16 

.36 

.37 

.006 

.053 

.23 

1.15 

4.20 

17 

.39 

.68 

.018 

.021 

.35 

.78 

5.19 

CRITICAL  POINTS 


Degrees  Fahr. 

Degrees  Fahr. 

No. 

No. 

Ac. 

Ar. 

Ac. 

Ar. 

1 

1472 

1256 

10 

1418 

1229 

2 

1463 

1274 

11 

1472 

1283 

3 

1508 

1265 

12 

1508 

1301 

4 

1454 

1274 

13 

1400 

860 

5 

1427 

1139 

14 

1400 

986 

6 

1418 

1301 

15 

1490 

986 

7 

1355 

941 

16 

1418 

770 

8 

1454 

617 

17 

1436 

482 

9 

1400 

788 

328 


STEEL  AND   ITS   HEAT   TREATMENT 


ANNEALED 


No. 

Annealing 
Temper- 
ature, 
Deg.  F. 

Tensile 
Strength, 
Lbs.  per 
Sq.  In. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elon- 
gation, 
in  2  Ins. 
Per  Cent. 

Reduc- 
tion 
of  Area, 
Per  Cent. 

Guillery 
Shock 
Test. 

Brinell 
Hard- 
ness 
Number. 

1 

1472 

80,690 

56,320 

26 

64.9 

133.8 

153 

2 

1472 

87,690 

61,160 

23 

55.7 

65 

170 

3 

1292 

105,400 

73,670 

22 

63.2 

112.1 

197 

4 

1652 

87,760 

50,200 

21.5 

53 

36,.  1 

168 

5 

1472 

90,600 

60,160 

20 

60.5 

115.7 

179 

6 

1382 

137,960 

72,960 

11 

23.8 

21.7 

210 

7 

1292 

114,650 

64,570 

17.5 

50.5 

8 

1112 

134,410 

126,440 

14.5 

59.2 

54.2 

250 

9 

1112 

115,930 

99,560 

18 

65.8 

101.2 

217 

10 

1382 

122,320 

71,120 

13 

45.5 

43.4 

220 

11 

1382 

135,830 

81,780 

14 

49.8 

54.2 

251 

12 

1112 

119,610 

83,910 

9.5 

46.7 

39.8 

273 

13 

1112 

163,850 

142,940 

13.5 

58.4 

137.4 

178 

14 

1652 

123,030 

75,100 

5.5 

9.8 

68.6 

232 

15 

1382 

142,510 

93,870 

6.5 

28 

39.8 

288 

16 

1112 

128,290 

119,610 

17 

62.4 

50.6 

225 

17 

1112 

147,210 

91,600 

14.5 

52.8 

47 

268 

NOTE:  It  will  be  noticed  that  in  a  number  of  instances  the  temperature  used  in  the 
above  annealing  is  under  the  Ac  point,  which  will  explain  the  high  tensile  results  obtained. 
Such  cases  do  not  represent  full  annealing. 


HEAT  TREATED 


No. 

Quenching 
Bath  and 
.Temper- 
ature, 
Deg.  F. 

Draw- 
ing 
Tem- 
pera- 
ture 
Deg.  F. 

Tensile 
Strength, 
Lbs.  per 
Sq.  In. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elon- 
gation 
in  2  Ins. 
Per 
Cent. 

Reduc- 
tion of 
Area, 
Per 
Cent. 

Guil- 
lery 
Shock 
Test. 

Brinell 
Hard- 
ness 
Num- 
ber. 

1 

Water,  1382 

204,500 

180,510 

10 

44.3 

68.6 

370 

2 

Oil,         1472 

225,010 

189,170 

7 

19.3 

47 

418 

3 

Oil,        1472 

572 

264,550 

214,820 

6.3 

42.7 

54.2 

412 

4 

Oil,        1562 

197,700 

173,520 

5 

15.4 

61.5 

328 

5 

Water,  1382 

201,970 

173,520 

10 

54 

72.3 

295 

6 

Oil,        1382 

932 

230,410 

221,880 

1.5 

4.3 

36.1 

388 

7 

Oil,        1562 

208,370 

183,350 

9.5 

51 

47 

343 

8 

Oil,        1472 

292,280 

289,440 

9.5 

36.3 

57.8 

425 

9 

Air,        1472 

221,020 

189,870 

8 

41 

54.2 

396 

10 

Water,  1472 

932 

190,580 

173,520 

6.5 

47.8 

79.5 

301 

11 

Cil,        1472 

572 

215,530 

202,250 

7 

42 

43.4 

395 

12 

Oil,        1472 

932 

210,500 

181,930 

6 

14.1 

32.5 

425 

13 

Water,  1472 

188,020 

168,970 

10 

56 

72.3 

286 

14 

Water,  1562 

183,190 

160,010 

10 

52.5 

57.8 

298 

15 

Water,  1382 

932 

187,640 

157,730 

7 

45.7 

72.3 

300 

16 

Air,        1562 

235,390 

225,860 

9 

24.5 

54.2 

402 

17 

Air,        1472 

32.5 

512 

CHROME  NICKEL  STEELS  329 

CHROME   NICKEL   STEEL   IN   AUTOMOBILE   CONSTRUCTION 

0.25  Carbon  and  under 

Principally  for  case-hardening  purposes,  such  as  bevel  driving 
and  transmission  systems,  steering-wheel  pivot  pins,  cam  rollers, 
push  rods,  and  similar  parts  which  must  not  only  have  a  hard 
exterior  surface,  but  possess  strength  as  well. 

0.25  to  0.35  Carbon 

Axles. — Steering  knuckles,  bolts,  pinions,  steering  pivots, 
spindles,  driving  shafts,  etc.,  gears  with  light  case,  drawn.  Gears 
hardened,  but  not  drawn. 

This  grade  of  chrome  nickel  steel  forges  and  machines  well, 
and  responds  to  heat  treatment  in  matter  of  strength  as  well  as  of 
toughness. 

0.35  to  0.45  Carbon 

Crankshafts. — Countershafts,  propeller  shafts,  live  axles, 
diving  shafts. 

This  grade  possesses  under  suitable  heat  treatment  a  high 
degree  of  strength  and  considerable  toughness.  Its  fatigue-resisting 
(endurance)  properties  are  extremely  high. 

0.45  to  0.55  Carbon 

Tempered  Gears. — This  grade  probably  gives  the  greatest 
possible  hardness  with  the  least  possible  brittleness  (in  combination) 
of  any  steel  for  transmission  purposes. 

MAYARI    CHROME   NICKEL   STEEL 

Mayari  steel  is  a  "  natural  alloy  "  steel  containing  from  .20  per 
cent,  to  .70  per  cent,  chrome  and  1.00  per  cent,  to  1.50  per  cent, 
nickel.  It  is  made  from  a  low-phosphorus  Cuban  ore  containing  the 
alloying  elements  chrome  and  nickel.  In  the  blast  furnace  the 
chrome  and  nickel  in  the  ore  are  reduced,  forming  a  natural  con- 
stituent of  the  iron.  By  means  of  the  duplex  process — Bessemer 
converter  and  open  hearth — the  iron  is  then  made  into  steel,  the 
chrome  and  nickel  pass  into  the  steel,  forming  a  natural  alloy,  with 
no  other  addition  of  these  elements  in  the  furnace  or  ladle  being 
necessary.  Mayari  steel  has  given  excellent  satisfaction  in  a  large 
number  of  cases,  although  it  undoubtedly  is  not  equal  to  synthetic 
chrome  nickel  steel  where  the  highest  quality  chrome  nickel  steel  is 
required. 


330 


STEEL  AND  ITS  HEAT  TREATMENT 


In  the  natural  or  forged  condition  Mayari  steel  has  from  8000 
to  10,000  Ibs.  per  square  inch  higher  tensile  strength  and  elastic 
limit  than  a  carbon  steel  of  the  same  carbon  content.  Like  all 
alloy  steels,  it  welds  with  more  or  less  difficulty  by  the  ordinary 
methods,  and  would  not  be  recommended  for  purposes  where  a 
welded  part  is  subject  to  great  strains.  By  careful  work  in  a  Thomp- 
son electric  welding  machine,  excellent  results  are  obtained,  so  that 
where  this  method  is  applicable  Mayari  steel  may  be  welded  satis- 
factorily. 

The  physical  properties  of  Mayari  steel,  heat  treated,  in  |  in. 
bars,  are  shown  in  Figs.  191,  192  and  193.  The  effect  on  the  physical 
properties  of  variation  in  the  size  of  the  piece  treated  is  indicated  in 
the  charts,  Figs.  194  and  195,  which  show  the  properties  of  heat- 
treated  rounds  from  1  in.  to  6  ins.,  and  J  in.  to  4J  ins.  diameter, 
respectively.  All  of  the  rounds  on  the  same  chart  were  from  the 
same  heat  of  steel.  These  were  treated  together  at  the  same  time  in 
exactly  the  same  manner.  The  first  chart  is  0.28  per  cent,  carbon, 
and  the  second  0.39  per  cent,  carbon;  both  grades  contained  0.45 
per  cent,  chrome  with  the  usual  nickel.  On  the  bars  over  2  ins. 
in  diameter  the  tests  were  taken  one-half  the  distance  from  the 
center  to  the  outside,  and  on  the  smaller  rounds  they  were  taken 
from  the  center. 

The  following  table  1  shows  the  approximate  difference  in  draw- 
ing temperatures  for  Mayari  steel  of  larger  sizes  than  those  given 
on  the  charts  of  Figs.  191  to  193.  When  it  is  desired  to  obtain  the 
same  elastic  limit  on  a  size  larger  than  f-in.  diameter,  find  the  draw- 
ing temperature  on  the  chart,  then  by  making  the  allowance  given 
in  the  table  below  for  the  size  desired,  the  proper  temperature  for 
this  elastic  limit  will  be  determined.  The  other  properties  will 
vary  from  those  on  the  chart  by  the  percentage  shown  in  the  table. 


Physical  Properties;    Per  Cent,  of  that  given  on  Charts  for 

J-in.  Rounds. 

Change  in 

Diameter. 

Drawing 

Temperature. 

Tensile 
Strength, 
Per  Cent. 

Elastic 
Limit, 
Per  Cent. 

Elongation, 
Per  Cent. 

Reduction  of 
Area, 
Per  Cent. 

fin. 

0 

100 

100 

100 

100 

2|  ins. 

-  90°  F. 

102 

100 

90 

96 

3£  ins. 

-135°F. 

110 

100 

87 

85 

4j  ins. 

-235°  F. 

122 

100 

80 

83 

iPenna.  Steel  Co. 


CHROME  NICKEL  STEELS 


331 


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332 


STEEL  AND   ITS  HEAT  TREATMENT 


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CHROME  NICKEL  STEELS 


333 


334 


STEEL  AND   ITS  HEAT  TREATMENT 


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10000 

0 


^^^j 


05  Jt 


•* 


FIG.  194. — Effect  of  Size  on  Physical  Properties  of  Mayari  Steel,  0.30  Carbon. 
Same  Analysis  and  Same  Treatment,     (Penna.  Steel  Co,) , 


C  280  000 
W270000 


S  140000 

a  130  ooo 

120  000 

"2  no  ooo 

a  100  000 
5  90000 
be  80000 
=  70000 


±l/a  rouad 


FIG.  195.— Effect  of  Size  on  Physical  Properties  of  Mayari  Steel,  0.40  Carbon. 
Same  Analysis  and  Treatment,     (Penna,  Steel  Co.) 


CHAPTER  XIV 
VANADIUM   STEELS 

THE  author  approaches  the  general  subject  of  vanadium  steels 
with  much  hesitation.  What  has  not  been  said  about  the  merits 
or  demerits  of  chrome  vanadium  versus  chrome  nickel  is  hardly 
worth  mentioning;  and  many  of  the  "  facts  "  (or  fancies?)  put  forth 
in  the  advocation  of  one  and  the  utter  condemnation  of  the  other 
should  not  be  mentioned.  Having  had  to  do  with  the  manufacture 
as  well  as  the  use  of  both,  he  feels  that  much  may  be  said  in  favor 
of  each,  but  that  neither  steel  is  the  "  one  and  only" — each  has  its 
specific  sphere  of  usefulness  and  most  advantageous  application. 

The  principal  effect  of  vanadium  additions  to  steel  is  its 
effect  upon  the  physical  characteristics  of  the  steel.  Like  most 
alloys,  vanadium  tends  to  give  a  finer  and  denser  structure 
than  that  ordinarily  obtained  in  straight  carbon  steels.  In  true 
vanadium  steels,  i.e.,  steels  in  which  vanadium  is  present  in 
definite  commercial  quantities,  the  general  action  of  vanadium  is 
similar  to  that  of  many  of  the  alloys  previously  discussed,  but  it 
also  presents  other  interesting  phenomena.  Vanadium,  in  the 
regular  steels  containing  about  0.12  to  0.20  per  cent,  vanadium, 
is  probably  present  in  both  the  ferrite  (similar  to  nickel)  and  also  as 
a  double  carbide  in  the  cementite  (similar  to  chrome).  In  support 
of  the  first  statement  that  vanadium  is  in  solid  solution  in  the  ferrite 
are  the  results  of  many  tests  which  appear  to  show  that  the  duc- 
tility is  higher  in  these  commercial  vanadium  steels  than  in  corre- 
sponding, steels  which  do  not  contain  vanadium.  This  is  based  on 
the  assumption,  generally  accepted,  that  the  nature  of  the  ferrite 
element  is  indicative,  to  a  large  degree,  of  the  ductility  of  the  steel. 

The  proof  direct  that  vanadium  forms  a  double  carbide  is  illus- 
trated by  steels  with  higher  percentages  of  contained  vanadium. 
Thus  steels  containing  0.2  per  cent,  carbon  and  up  to  0.7  per  cent, 
vanadium,  or  0.8  per  cent,  carbon  and  0.5  per  cent,  vanadium,  are 
normally  pearlitic;  but  any  increase  in  the  vanadium  content  over 
these  limits  will  produce  a  characteristic  double  carbide  component. 

335 


336  STEEL  AND   ITS  HEAT  TREATMENT 

From  these  limiting  ratios  of  carbon  and  vanadium  it  is  evident 
that  vanadium  has  a  powerful  influence  upon  the  transformation 
ranges — more  so,  indeed,  than  any  of  the  common  alloying  elements. 
This  also  goes  to  show  the  reason  why  only  small  quantities— 
0.25  per  cent,  vanadium  or  under — are  necessary  to  produce  a 
noticeable  effect. 

As  a  general  proposition,  any  alloy  which  tends  to  form  a  cemen- 
titic  compound  in  steel  also  has  the  tendency  to  require  a  higher 
temperature  for  quenching  in  order  to  bring  the  steel  as  a  whole 
into  a  state  of  equalization.  This  was  found  to  be  true  in  the  case 


FIG.  196. — Chrome  Vanadium  Steel,  Type  A,  Oil  Treated  at  the  Same  Tempera- 
tures Used  for  a  Corresponding  Chrome  Nickel  Steel.     X60.     (Bullens.) 

of  chrome,  and  it  is  also  true  of  vanadium  steels.  A  study  of  the 
heat-treatment  data  subsequently  given  will  show  that  vanadium 
alloy  steels  give  the  best. results  with  an  apparently  abnormally  high 
quenching  temperature,  or  at  about  1560°  to  1600°  F.  for  the  me- 
dium carbon  grades.  This  point  is  also  illustrated  by  the  photo- 
micrograph in  Fig.  196;  which  illustrates  the  structure  of  a  rolled 
plate  of  "  Type  A  "  chrome  vanadium  steel  oil  treated  at  the  tem- 
peratures best  suited  for  a  chrome  nickel  steel  of  the  same  carbon 
and  manganese  content.  From  the  structure  thus  shown  it  is 
evident  that  the  steel  as  a  whole  has  not  been  equalized  at  the 
temperature  for  hardening  (1500°  F.)  which  was  used,  since  the 


VANADIUM  STEELS  337 

ferrite  (white)  is  still  segregated  and  tends  to  follow  the  lamellar 
structure  of  the  original  steel. 

On  the  other  hand,  if  we  follow  out  the  characteristics  peculiar 
to  most  alloy  steels  of  a  carbide  nature,  we  would  expect  that  the 
vanadium  steels  would  be  inherently  more  sensitive  to  prolonged 
heating  or  rapid  cooling.  Now  while  it  is  true  that  steels  containing 
vanadium  will  give  a  greater  depth  of  hardness  upon  suitable  quench- 
ing than  will  some  steels  of  a  ferritic  nature  (such  as  nickel  steels), 
it  does  not  appear  to  be  true  that  vanadium  abnormally  increases 
the  sensitiveness  of  the  steel  to  prolonged  heating.  This  appears 
to  be  one  of  the  anomalies  of  vanadium  steels. 

Viewed  from  the  standpoint  of  physical  test  values,  vanadium 
requires  the  presence  of  another  alloy  as  an  "  intensifier,"  in  order 
that  the  full  effect  and  influence  of  the  vanadium  additions  may  be 
felt.  Just  as  chrome  greatly  intensifies  the  influence  of  nickel  in 
steel,  so  chrome  also  seems  to  bring  out  the  latent  capabilities  of 
vanadium,  but  to  an  even  greater  extent.  Thus  the  majority  of 
the  vanadium  steels  now  in  commercial  use  are  of  the  chrome  vana- 
dium type. 

The  predominant  note  which  is  always  sounded  when  speaking 
or  writing  about  chrome  vanadium  or  vanadium  steels  is  that  of 
increased  dynamic  strength.  There  is  little  doubt  but  that  vana- 
dium greatly  increases  the  dynamic  strength  in  comparison  with 
that  of  a  corresponding  straight  carbon  steel.  Upon  the  relative 
merits,  as  regards  dynamic  strength,  of  chrome  vanadium  and 
chrome  nickel  steels,  we  have  commented  under  the  latter  steels. 
Extensive  tests  made  by  the  author  to  determine  dynamic  strength 
have  led  to  varying  results,  and  he  deems  it  best  to  leave  the 
subject  with  the  warning  given  in  the  opening  paragraph  of  this 
chapter. 

The  following  equations  connecting  maximum  strength,  Brin- 
ell  hardness  number  and  scleroscope  hardness  number  have  been 
computed  1  from  several  hundred  tests  made  with  chrome  vanadium 
of  different  carbon  content  and  heat  treated  to  bring  out  all  pos- 
sible physical  properties: 


(1)  M  = 

(2)  M=4.2    £-21. 

(3)  £  =  5.5    S+27. 

R.  R.  Abbott,  A.  S.  T.  M.,  Vol.  XV,  Part  II,  1915,  p.  43  et  seq. 


338 


STEEL  AND   ITS  HEAT   TREATMENT 


M  =  maximum  strength  in  units  of  1000  Ibs.  per  sq.  in. 
B  =  the  Brinell  hardness  number. 
S  =  the  scleroscope  hardness  number. 

The  maximum  strength  corresponding  to  different  Brinell  val- 
ues as  determined  by  equation  (1)  for  these  steels  is  as  follows: 


Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

100 

42,000 

350 

219,000 

150 

77,000 

400 

255,000 

200 

113,000 

450 

290,000 

250 

148,000 

500 

326,000 

300 

184,000 

550 

361,000 

The  maximum  strength  corresponding  to  different  scleroscope 
values  as  determined  by  equation  (2),  and  the  corresponding  Brin- 
ell numbers  as  determined  by  equation  (3),  for  these  steels,  are  as 
follows : 


Scleroscope. 

Maximum  Strength, 
Lbs.  per  Sq.  In. 

Brinell. 

20 

63,000 

137 

30 

105,000 

192 

40 

147,000 

247 

50 

189,000 

302 

60 

231,000 

357 

70 

273,000 

412 

80 

315,000 

467 

90 

357,000 

522 

100 

399,000 

577 

Static  test  results  1  upon  various  "  types  "  of  vanadium  steels 
follow: 


In  part  by  the  American  Vanadium  Co.,  Pittsburgh,  Pa. 


VANADIUM   STEELS 


339 


TYPE  "  A  "  CHROME- VANADIUM  STEEL 
Tests  from  Small  Sections 

Carbon 26%  Manganese 

Chromium 92%  Silicon 

Vanadium 20% 


.48% 
.06% 


Treatment. 

Tensile 
Strength. 

Elastic 
Limit. 

Elongation 
in  2  Ins.,%. 

Reduction 

of  Area,  %. 

As  rolled           

132,000 

110,000 

19.0 

51.5 

Annealed  1475°  F  
Oil  tempered: 
1650  °-l  155°  F  . 

83,700 
133,000 

61,000 
99,020 

34.8 
30  0 

66.4 
69.9 

1650-1110  
1650  -1020  
1650  -  930 

137,000 
141,500 
162  700 

112,000 
123,000 
146,250 

20.0 
18.0 
15  0 

61.0 
63.5 
57.0 

1650  -  840 

177,500 

151,500 

14.0 

53.0 

1650  -  750  

183,500 

155,000 

13.0 

51.0 

1560-1155  
1560  -1110 

131,000 
133,000 

100,000 
108,400 

28.0 
17.5 

67.0 
65.4 

1560  -1020  

137,500 

112,750 

21.0 

64.5 

1560-930  ...... 
1560  -  840  
1560  -  750  

156,800 
171,100 
173,900 

138,440 
147,150 
149,800 

16.5 
15.0 
13.0 

59.8 
61.0 
57.0 

Water  tempered: 
1650  °-l  155°  F.. 

156,000 

133,000 

18  0 

62  5 

1650  -1110  . 

160,900 

149,700 

16.0 

60.4 

1650  -1020  
1650-930  
1650  -  840  
1560-1155  
1560-1110  
156C-1020  
1560  -  930 

167,800 
183,200 
204,800 
153,050 
156,500 
166,800 
176,950 

151,000 
166,800 
176,200 
136,600 
146,300 
149,100 
165,000 

12.0 
12.5 
12.5 
27.0 
17.0 
14.0 
14  0 

53.6 
56.5 
54.5 
60.0 
61.0 
58.9 
59  0 

1560  -  840 

201,800 

172,800 

12.5 

54.5 

Tests  from  Medium  Sections 

Carbon 23%  Manganese 58% 

Chromium 82%  Silicon 105% 

Vanadium 17% 


Stock. 

Treatment. 

Tensile 
Strength. 

Elastic 
Limit. 

Elongation 
in  2  Ins.,  %. 

Reduction 
of  Area,  %. 

Oil  tempered: 

2^-in 

1650°-1050°  F.. 

125,730 

108,950 

19.0 

60  4 

2i-in 

1650  -1050 

124,160 

106,000 

20.0 

60.6 

2f-in. 

1650  -1050      

122,740 

104,750 

19.5 

57.0 

2f-in. 

1650  -1050      

126,700 

111,500 

17.0 

53.0 

2|-in. 

1650-1050      

121,080 

106,500 

18.0 

60.7 

2|-in. 

1650-1050      

124,130 

107,000 

18.5 

61.1 

340 


STEEL  AND  ITS   HEAT  TREATMENT 


Test  from  6-in.  Tender  Axle 

Carbon 29%  Manganese 28% 

Chromium 1.00%  Silicon 06% 

Vanadium 20% 


Treatment. 

Tensile 
Strength. 

Elastic 
Limit. 

Elongation 
in  2  Ins.,%. 

Reduction 
of  Area,%. 

Water  tempered: 
1690°-1155°F  

115,000 

90,000 

21.0 

55.0 

Tests  from  Locomotive  Driving  Axles,  10  7ns.  Diameter 
Average  Test  of  287  Heat-treated  Axles 

Carbon 35%  Manganese 50% 

Chromium 90%  Vanadium 22% 

Elastic  limit,  pounds  per  square  inch 81,600 

Tensile  strength,  pounds  per  square  inch 108,890 

Elongation  in  2  ins.,  per  cent 21 . 75 

Reduction  of  area,  per  cent 58 . 75 

TYPE  "  D  "  CHROME  VANADIUM  STEEL 
Tests  on  Small  Sections 

Carbon 50%  Manganese 92% 

Chromium 1 . 02%  Silicon 065% 

Vanadium 20% 


Treatment. 

Tensile 
Strength. 

Elastic 
Limit. 

Elongation 
in  2  Ins.,%. 

Reduction 
of  Area,  %. 

Brinell 
Hardness 
No. 

As  rolled  

153,350 

124,450 

12.5 

37.0 

286 

Annealed  1475°  F  

103,440 

63,660 

25.8 

61.5 

187 

Oil  tempered: 

1650°-11100  F  

186,800 

170,000 

15.5 

45.2 

340 

1650  -1020       

201,150 

186,100 

13.0 

45.5 

364 

1650  -  930       

209,800 

192,200 

12.5 

42.5 

364 

1650  -  840 

227,040 

217,360 

10.0 

35.5 

402 

1650  -  750       

264,500 

239,700 

6.5 

17.0 

444 

1600-1110       

186,100 

161,200 

13.5 

45.5 

340 

1600  -1020       

205,500 

187,000 

12.0 

45.0 

340 

1600-  930       

214,050 

203,600 

11.5 

43.0 

380 

1600  -  840       

237,500 

221,000 

10.0 

29.5 

418 

1560-1020       

197,100 

187,100 

12.5 

45.0 

340 

1560  -  930       

214,270 

201,400 

11.5 

36.0 

418 

1560  -  840       

234,150 

215,850 

9.0 

28.5 

418 

1560  -  750       

261,850 

.  240,000 

7.0 

22.0 

418 

1520-1020       

183,500 

177,250 

14.5 

47.5 

340 

1520  -  930       

215,450 

193,100 

12.0 

41.5 

387 

1520  -  840       

237,750 

213,400 

10.0 

35.5 

387 

1520  -  750       

260,500 

240,000 

8.0 

24.0 

444 

VANADIUM   STEELS 


341 


Fig.  197. — Effect  of  Mass  upon  the  Hardness  of  Chrome  Vanadium  Steel. 

(Matthews  &  Stagg.) 


The  effect  of  mass  upon  the  hardness  of  steel  of  this  type  is 
shown  in  Fig.  197. l 


TYPE  "  G  "  CHROME  VANADIUM  STEEL 

Carbon 60%  Manganese 54% 

Chromium 88%  Silicon 175% 

Vanadium 19% 


Treatment. 

Tensile 
Strength. 

Elastic 
Limit. 

Elongation 
in  2  ins.,%. 

Reduction 
of  Area,%. 

Brinell 
Hardness 
No. 

Oil  tempered: 

| 

1650°-1110°F  

205,190 

179,300 

13.0 

37.0 

402 

1650  -  930       

240,400 

220,000 

10.0 

28.3 

477 

1650-750       

273,000 

248,660 

8.0 

27.3 

532 

From  Matthews  and  Stagg,  "  Factors  in  Hardening  Tool  Steel." 


342 


STEEL  AND   ITS  HEAT  TREATMENT 


NICKEL  VANADIUM  STEEL 


Carbon 29% 

Nickel 3.41% 

Vanadium.  . 


Magnanese .  .  . 
Silicon 

20% 


.45% 
.090% 


Treatment. 

Tensile 
Strength. 

Elastic 
Limit. 

Elongation 
in  2  ins.,%. 

Reduction 
of  Area,  %. 

Annealed  800°  C  

107,300 

73,000 

23.5 

55.5 

Oil  tempered: 

1600°-1160°F  

148,300 

126,250 

18.0 

58.0 

1600-1110       

150,000 

128,500 

17.5 

57.4 

1600  -1020       

151,500 

132,500 

16.0 

56.9 

1600-930       

162,000 

144,200 

14.5 

52.6 

1600  -  840       

178,200 

157,210 

13.0 

52.7 

1600-  750       

193,200 

163,000 

12.0 

50.2 

1520-1160 

137,700 

123,000 

16.0 

59.0 

1520  -1110 

140,700 

125,500 

17.5 

54.2 

1520  -1020       

148,100 

126,800 

16.5 

55.0 

1520  -  930       

154,900 

135,000 

15.5 

57.2 

1520  -  840       

165,800 

146,500 

14.0 

55.2 

1520  -  750       

181,000 

162,800 

14.0 

53.5 

Water  tempered: 

1600°-!  160°  F  

148,000 

126,700 

18.5 

58.1 

1600-1110       

153,800 

133,100 

15.0 

58.8 

1600  -1020       

156,300 

136,500 

14.0 

54.5 

1600-930       

161,200 

146,700 

14.5 

56.4 

1600-840       

186,400 

173,300 

13.0 

52.7 

1600  -  750       

195,200 

176,580 

12.0 

52.2 

1520-1160       

139,800 

128,570 

18.5 

59.7 

1520  -1110 

146,000 

132,250 

14.0 

57.5 

1520  -1020       

154,600 

133,900 

15.5 

56.3 

1520  -  930       

160,400 

144,600 

15.0 

51.7 

1520-840       

184,500 

176,750 

13.0 

53.0 

1520  -  750 

199,300 

182,700 

12.0 

50.0 

VANADIUM  STEELS 


343 


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CHAPTER  XV 
MANGANESE,  SILICON  AND  OTHER  ALLOY  STEEL 

MANGANESE    STEELS 

THE  term  "  manganese  steel,"  by  commercial  usage,  generally 
refers  to  steels  with  that  high  percentage  of  manganese  which  will 
cause  the  metal  to  become  austenitic  under  the  conditions  of  ordinary 
cooling  or  suitable  heat  treatment.  But  before  proceeding  to  a 
discussion  of  such  steels,  it  is  desirable  to  amplify  the  remarks  we 
have  previously  made  upon  the  subject  of  pearlitic  manganese 
steels. 

PEARLITIC    MANGANESE    STEELS 

It  is  a  well-known  fact  that  manganese  in  these  steels  adds  con- 
siderably to  the  tensile  strength;  this  beneficial  effect  is  further 
dependent  upon  the  percentage  of  carbon,  as  has  been  previously 
shown.  On  the  other  hand,  the  effect  of  manganese  in  normally 
pearlitic  manganese  steels  upon  the  fragility  of  the  steel  is  a  more  or 
less  undetermined  factor.  Many  persons  have  undoubtedly  con- 
fused the  subject  of  inherent  brittleness  or  non-resistance  to  shock 
with  the  results  caused  by  the  sensitiveness  of  these  steels  to  certain 
heat-treatment  methods.  They  have  assumed,  because  a  piece  of 
steel  with  1  to  2  per  cent,  manganese  may  have  cracked  on  drastic 
water  quenching,  that  the  steel  was  "  brittle,"  when,  as  a  matter  of 
fact,  this  result  was  probably  due  to  reasons  entirely  apart  from  the 
dynamic  strength  of  the  metal.  Thus  it  may  be  said  that  this  situa- 
tion has  led  to  the  belief  that  manganese  contributed  an  embrittling 
effect  to  the  steel.  Even  assuming  that  such  an  influence  may 
exist  in  the  case  of  very  high-carbon  steels,  it  distinctly  has  not  been 
proven  to  be  true  of  the  hypo-eutectoid  steels.  In  fact,  there  is 
now  considerable  evidence  which  tends  to  show  that  the  lower- 
carbon  pearlitic  manganese  steels,  when  properly  made  and  suitably 
heat  treated)  are  not  brittle.  If  the  steel  is  made  in  small  heats,  has 
been  thoroughly  refined,  and  with  the  elimination  of  impurities  to 

344 


MANGANESE,   SILICON   AND   OTHER  ALLOY  STEELS   345 

a  minimum,  the  author  believes  that  a  great  deal  may  be  accom- 
plished with  such  steels. 

With  steel  made  by  the  ordinary  open-hearth  process,  it  should  be 
remembered  that  the  presence  of  any  considerable  amount  of  man- 
ganese, such  as  1  per  cent,  or  more,  has  the  tendency  to  increase  the 
sensitiveness  of  the  steel  in  its  response  either  to  prolonged  heating 
at  temperatures  above  the  critical  range,  or  to  rapid  cooling  from 
such  temperatures.  Thus  high  temperatures  of  annealing  will 
increase  the  grain  size  very  rapidly;  while  high  carbon  may  cause 
the  steel  to  fracture  when  water  quenched. 

On  the  other  hand,  certain  manganese  steels  with  1.5  to  2  per 
cent,  manganese  and  a  considerable  carbon  content,  made  in  the 
electric  furnace,  have  shown  wonderful  mechanical  properties,  and, 
in  addition,  will  stand  a  tremendous  amount  of  abuse  in  their  thermal 
treatment  without  any  great  ill  effects.  Granting  that  the  electric 
furnace  is  capable  of  producing  a  higher  grade  of  steel  than  other 
processes  now  in  use,  it  must  nevertheless  be  evident  that  a  large 
proportion  of  the  merits  of  these  pearlitic  manganese  steels  must 
be  due  to  the  inherent  influence  of  the  manganese  itself. 

In  treating  pearlitic  manganese  steels  it  should  be  remembered 
that  each  0.1  per  cent,  manganese  will  lower  the  critical  range  on 
heating  by  about  5°  to  6°  F.,  so  that  lower  temperatures  may,  and 
in  most  cases  should,  be  used  for  their  hardening  or  full  annealing. 
In  general,  the  effect  of  manganese  on  the  critical  ranges  is  about 
twice  that  of  nickel. 

HIGH-MANGANESE    STEELS 

In  general,  the  requirements  for  producing  a  commercial  mangan- 
ese steel  necessitate  a  manganese  content  of  about  6  or  8  per  cent, 
to  20  per  cent.,  in  combination  with  the  proper  amount  of  carbon. 
Below  the  lower  limits  given,  the  steel,  even  by  the  most  suitable 
treatment,  may  be  characterized  by  the  presence  of  weak  and 
brittle  martensite.  The  upper  limits  are  determined  by  the  cost 
of  the  manganese  additions,  and  further,  by  the  again  predominating 
influence  of  the  carbon  content  (when  the  manganese  rises  to  around 
20  per  cent.),  which  will  make  the  steel  stiff  and  brittle  when  cold. 
Most  manganese  steels  will  have  about  11  or  12  per  cent,  manganese 
and  about  1.0  to  1.2  per  cent,  carbon. 

Recent  research  work  along  the  lines  of  determining  the  proper 
combination  of  carbon  and  manganese  has  greatly  widened  the 
commercial  range  for  the  manganese  content,  so  that  the  more 


346  STEEL  AND   ITS  HEAT  TREATMENT 

recent  steels  have  the  tendency  toward  a  percentage  of  manganese 
lower  than  that  originally  thought  necessary.  Similarly,  the  field 
for  the  use  of  high  manganese  steels  has  also  been  considerably 
broadened.  Above  all,  however,  the  peculiar  merit  of  these  steels 
lies  in  the  resistance  to  abrasive  wear,  in  combination  with  suffi- 
cient strength  and  ductility.  In  this  regard,  manganese  steels 
appear  to  resist  the  abrasive  wear  characteristic  of  heavy  impacts 
of  hard  substances  better  than  that  caused  by  the  sliding  attrition 
of  hardened  parts,  or  like  that  of  an  abrasive  wheel. 

Aside  from  the  dynamic  strength,  the  selection  of  a  manganese 
steel  for  any  specific  work  depends  upon  the  correlation  of  wear- 
ing qualities  and  static  properties.  In  general,  and  in  connection 
with  a  maximum  wear  resistance,  it  may  be  said  that  the  most 
ductile  steel  which  will  give  an  elastic  limit  sufficiently  high  to  avoid 
distortion  in  service  will  be  best.  And  these,  in  turn,  depend  upon 
the  proper  combination  of  carbon  and  manganese.  Thus  a  steel 
with  9  to  11  per  cent,  manganese  and  the  proper  amount  of  carbon 
will  have  a  higher  elastic  limit  than  a  steel  with  over  11  per  cent, 
manganese.  Again,  steel  with  11  per  cent,  manganese  and  1.10 
per  cent,  carbon  will  have  a  higher  elastic  limit  than  a  steel  with 
15  per  cent,  manganese  and  0.8  per  cent,  carbon.  With  high  man- 
ganese and  low  carbon,  steels  quenched  in  water  from  1830°  F.  will 
give  a  low  elastic  limit  and  a  flow  of  metal  which  may  prove  excessive 
for  many  duties.  A  great  deal  also  depends  upon  a  suitable  heat 
treatment  of  the  steel. 

As  might  be  expected  from  our  knowledge  of  the  influence  of 
the  rate  of  cooling  upon  the  structure  of  high-alloy  steels,  the  physical 
properties  of  these  high-manganese  steels  are  greatly  modified  by 
the  method  of  casting,  the  size  of  the  casting,  and  the  mechanical 
elaboration.  The  first  two  factors  in  particular  have  a  great  influence 
upon  the  toughness  of  the  metal.  The  average  tests  of  commercial 
manganese  steels  with  about  11  or  12  per  cent,  manganese  and  a 
little  over  1.0  per  cent,  carbon  will  give  approximately  the  following: 


Condition  of  the  metal. 

Tensile  Strength, 
Lbs.  per  Sq.  In. 

Elastic  Limit, 
Lbs.  per  Sq.  In. 

Elongation, 
%  in  2  Irs. 

Cast 

82000 

45000 

30 

Rolled  

135,000-140,000 

60  000-70  000 

30-40 

Forged  

142,000 

55  000 

38 

The  elastic  limit  of  some  sections  as  rolled  may  even  go  as  high  as 
75,000  Ibs.  per  square  inch;   the  proper  heating  and  working  of  the 


MANGANESE,   SILICON  AND   OTHER  ALLOY  STEELS   347 

metal  plays  a  very  important  part  in  the  results  to  be  obtained  on 
physical  test. 

A  common  specification  for  manganese  steel  rails  is  as  follows: 
Chemical : 

Carbon,  per  cent 0. 95  to    1 . 15 

Manganese,  per  cent 10       to  13 

Silicon,  per  cent 0. 20  to    0.40 

Phosphorus,  per  cent under  0. 10 

Sulphur,  per  cent under  0. 06 

Physical : 

Tensile  strength,  Ibs.  per  sq.  in. .  . 100,000 

Elastic  limit,  Ibs.  per  sq.  in 55,000 

Elongation  in  2  ins.,  per  cent 20 

Small  amounts  of  chrome  are  sometimes  added  to  increase  the 
elastic  limit,  so  that  in  rolled  sections  the  elastic  limit  will  often  be 
as  high  as  85,000.  It  is  stated  that  the  resistance  to  shock  is  not 
apparently  lowered  by  the  addition  of  chrome  up  to  1  per  cent., 
but  with  chrome  above  0.5  per  cent,  the  elongation  is  rapidly  de- 
creased, and  with  chrome  above  1  per  cent,  the  elongation  falls 
below  20  per  cent,  in  2  inches. 

The  heat  treatment  of  high-manganese  steels  presents  a  most 
important  phase  in  connection  with  the  successful  application  of 
these  steels.  Incorrect  treatment  is  responsible  for  many  of  the 
failures  which  have  been  registered  against  manganese  steels,  and 
usually  has  been  caused  either  by  a  mistaken  idea  of  the  particular 
structure  best  suited  to  the  specific  work  in  hand,  or  by  a  lack  of 
sufficient  knowledge  of  the  mechanics  of  the  austenite  transforma- 
tion. The  use  of  the  microscope,  and  a  judicious  consideration 
and  application  of  the  results  obtained,  are  probably  the  best  means 
of  solving,  a  given  problem  in  connection  with  heat-treatment 
adjustments. 

This  thermal  treatment  for  the  majority  of  these  steels  involves 
two  distinct,  though  correlated  factors:  (1)  The  change  of  grain 
size,  and  (2)  the  relationship  of  austenite  and  carbide,  with  or  with- 
out the  presence  of  martensite.  As  the  principal  manganese  steels 
now  used  in  commercial  practice  do  not  naturally  contain  mar- 
tensite, nor  is  it  generally  wanted,  its  consideration  may  be  omitted. 

The  necessity  for  the  first  requirement  should  be  obvious:  com- 
mercial high-manganese  steel  as  cast  is  fundamentally  austenitic; 


348 


STEEL  AND   ITS  HEAT  TREATMENT 


the  crystals  are  often  excessively  large,  and  in  many  instances  form 
a  weak,  columnar  structure.  It  is  evident  that  such  a  steel,  for 
many  purposes,  will  be  entirely  unsatisfactory. 

On  the  other  hand,  many  high-manganese  steels  as  forged,  are 
characterized  by  an  exceedingly  fine,  almost  chalky  structure,  and 
yet  may  be  very  brittle.  If  a  bar  of  manganese  steel  should  be 
heated  to  1800°  F.,  and  one  half  be  allowed  to  cool  slowly  and  the 
other  half  quenched  in  water,  both  ends  will  have  a  comparatively 
fine  structure  or  grain  size,  yet  the  slow-cooled  end  will  have  but 
2  to  4  per  cent,  elongation  as  against  50  to  60  per  cent,  elongation 
in  the  quenched  end. 


Fig.  198. — Commercial  Manganese  Steel  Annealed  at  1750°  F. 
X100.     (Bullens.) 

Although  annealing  will  effect  a  change  in  the  grain  size,  to 
a  greater  or  lesser  extent,  it  will  also  have  a  far-reaching,  vastly 
more  important,  and  injurious  result — and  we  intentionally  omit  any 
reference  to  the  tendency  which  certain  compositions  might  have  to 
become  martensitic  on  very  slow  cooling.  This  effect  of  annealing 
is  due  to  the  formation,  on  very  slow  cooling,  of  the  maximum 
amount  of  carbide — an  extremely  hard  and  brittle  manganitic 
cementite  rejected  by  the  austenite,  and  which  forms  as  a  weak 
membrane  around  the  austenite  grains,  as  spines  and  needles,  or 
in  other  characteristic  manner.  This  is  shown  in  the  photomicro- 
graph in  Fig.  198,  taken  from  a  rolled  commercial  manganese  steel 


MANGANESE,   SILICON  AND  OTHER  ALLOY  STEELS  349 


and  then  annealed  at  1750°  F.  The  effect  of  annealing  upon  the 
physical  properties  is  shown  by  the  following  table  of  results, 
obtained  from  tests  upon  annealed  manganese  steels: 

HIGH-MANGANESE  STEELS,  ANNEALED. 


Carbon, 
Per  Cent. 

Manganese, 
Per  Cent. 

Tensile 
Strength, 
Lbs.  per  Sq.  In. 

Bias  tic 
Limit, 
Lbs.  per  Sq.  In. 

Elongation 
in  2  Ins., 
Per  Cent. 

Reduction 
of  Area. 
Per  Cent. 

0.95 

10.07 

94,600 

67,500 

1 

1.4 

1.00 

11.21 

99,950 

77,660 

1 

0.75 

1.07 

13.38 

103,670 

62,350 

3 

3.4 

Annealing  these  high-carbon  manganese  steels  is  obviously 
illogical,  and  it  is  the  carbide  which  is  the  main  source  of  the  diffi- 
culty. And  yet  a  large  proportion  of  the  peculiar  and  distinctive 
wearing  qualities  of  these  steels  is  probably  due  to  the  manganitic 
carbide,  although  in  a  form  other  than  that  just  described. 

On  heating  to  a  high  temperature  the  carbide  membrane  produced 
by  slow  cooling,  or  the  carbide  segregations,  are  gradually  taken 
into  solution  by  the  austenite;  and  by  rapid  cooling  from  that  same 
temperature  the  carbide  is  more  or  less  prevented  from  reprecip- 
itating,  and  especially  from  taking  on  that  enweakening  structure 
(i.e.,  as  a  membrane  or  segregations)  previously  mentioned.  Since 
the  carbide  originally  formed  in  casting  is  very  sluggish  in  its  response 
to  heat  in  being  absorbed  by  the  austenite  (as  is  also  a  character- 
istic more  or  less  marked  in  all  hyper-eutectoid  steels),  and  as  the 
equalization  of  the  steel  as  a  whole  also  takes  place  slowly,  a  high 
temperature  is  necessary.  Further,  the  temperature  must  also  be 
high,  and  the  cooling  be  effected  very  rapidly — such  as  water  quench- 
ing— to  retain  the  carbide  in  solution.  Such  a  treatment  will  give 
the  most  ductile  steel.  The  temperature  required  is  generally  not 
less  thanJ830°  F.  Thus  a  water  quenching  from  1830°  F.  of  the 
annealed  steels  previously  given  will  show  a  tensile  strength  of  about 
135,000  to  145,000  Ibs.  per  square  inch,  with  an  elongation  of  50 
to  60  per  cent. 

On  the  other  hand,  by  varying  the  factors  of  temperature, 
duration  of  heating,  and  rate  of  cooling,  it  is  possible  to  obtain  phys- 
ical properties  covering  a  wide  range.  The  static  strength  and 
ductility  are  largely  governed  by  the  amount  of  the  original  free  car- 
bide which  is  taken  into  solution  and  there  retained  by  water  quench- 
ing. Thus  the  properties  of  the  steel,  looking  at  the  results  of  heat 


350  STEEL  AND  ITS  HEAT  TREATMENT 

treatment  from  this  point  of  view,  may  be  varied  from  that  charac- 
teristic of  the  steel  as  cast,  rolled,  or  forged,  to  that  indicative  of  a 
full  "  water  toughening." 

There  is  another  possible  development  of  scientific  heat  treat- 
ment, however,  which  the  author  believes  has  not  been  fully  noted— 
not  even  by  some  manufacturers  of  high-manganese  steel  itself. 
And  by  this  is  meant  a  treatment  which  will  "  spheroidalize  "  the 
carbide.  The  application  of  such  a  process  as  applicable  to  straight 
carbon  steels,  and  the  superior  qualities  thereby  resulting  in  resist- 
ance to  wear  as  relative  to  case-hardening  work,  have  been  discussed 
under  Chapter  VII.  A  microscopic  examination  of  many  properly 
treated  high-tungsten  steels  will  show  a  similar  spheroidalizing 
action,  and  to  which  many  of  the  mechanical  properties  of  these 
steels  may  be  due — even  though  the  exact  relation  of  cause  and  effect 
may  not  be  known.  And  so  certain  processes  may  be  worked  out 
(or  so  the  author  is  led  to  believe)  for  these  high-manganese  steels, 
in  which  resistance  to  wear  is  the  main  result  desired.  Based  upon 
theory,  and  upon  partly  developed  experiments,  the  author  offers 
the  above  merely  as  suggestions. 

In  conclusion,  the  author  would  call  attention  to  the  fact  that 
high-manganese  steel  has  no  critical  or  transformation  points  or 
ranges.  Thus  while  in  the  ordinary  steels  the  heat  treatment  is 
more  or  less  guided  by  such  temperatures,  in  high-manganese  steels 
the  only  criterion  of  proper  temperatures  is  the  relation  of  the  car- 
bide to  the  physical  properties:  the  absorption,  with  or  without  the 
precipitation,  of  such  carbide,  is  the  underlying  basis  for  heat-treat- 
ment adjustment. 

SILICON  AND    SILICO-MANGANESE    STEELS 

The  use  of  silicon  in  commercial  steels  is  practically  confined 
to  two  classes:  (1)  a  medium  carbon  and  about  1.50  per  cent,  silicon, 
for  use  in  tempered  gears  and  springs — known  as  silico-manganese 
steel;  and  (2)  a  nearly  carbonless  steel  with  up  to  3.50  per  cent, 
silicon,  for  use  in  electrical  apparatus. 

The  manufacture  of  silico-manganese  steels  in  the  open  hearth 
must  be  carefully  watched  on  account  of  their  great  tendency  to 
piping  and  segregation,  and  a  large  discard  must  be  made  in  the 
cropping  of  the  ingots  to  insure  good  steel.  Silico-manganese  steels 
have  considerable  popularity  among  the  foreign  automobile  manu- 
facturers; their  use  in  this  country,  however,  is  usually  limited  to 
the  lower-priced  cars.  Although  the  lower  cost  favors  their  use, 


MANGANESE,   SILICON  AND  OTHER  ALLOY  STEELS   351 

their  great  sensitiveness  to  heat  treatment  and  feeble  resistance  to 
shock  limits  their  field  of  usefulness.  But  when  handled  with  great 
care  the  silico-manganese  (and  also  the  silico-chrome)  steels  will 
give  good  results  in  works  well  equipped  for  obtaining  accurate 
results  in  their  heat-treatment  operations.  The  temperature  limits 
for  quenching  are  narrower  than  for  most  alloy  steels,  and  the  steel 
responds  altogether  too  quickly  to  variations  in  heating  and  cooling. 
Although  these  steels  will  give  high  static  test  results  upon  suitable 
treatment,  the  brittleness  which  is  inherent  to  this  type  of  steel 
usually  proves  the  governing  factor. 

A  typical  American  analysis  for  silico-manganese  steel  for  gears 
and  springs  is  as  follows: 

Carbon,  per  cent 0.43  to  0. 53 

Manganese,  per  cent 0. 50  to  0. 70 

Silicon,  per  cent 1 . 25  to  1 . 50 

Upon  suitable  treatment,  usually  a  quenching  in  oil  from  about  1550° 
to  1600°  F.,  followed  by  tempering  to  suit  the  requirements,  the  fol- 
lowing results  are  representative : 

Tensile  strength,  Ibs.  per  sq.  in 195,000  to  230,000 

Elastic  limit,  Ibs.  per  sq.  in 175,000  to  220,000 

Elongation,  per  cent,  in  2  ins 12  to  8 

The  tables  (taken  from  a  work  by  Revillon)  on  page  352  give  some 
characteristic  French  and  German  gear  steels  of  the  silicon-man- 
ganese type,  together  with  their  physical  properties.  Steel  No.  6 
(a  straight  carbon  steel)  is  given  for  comparison,  and  was  designed 
to  do  away  with  the  reheating  or  tempering  necessary  with  the 
high-silicon  steels;  it  is  reported  that  tests  on  the  untempered  gears 
gave  very  satisfactory  results. 

As  a  side-light  on  the  effect  of  certain  treatments  of  pieces  of 
large  section  of  silico-manganese  steel,  the  following  may  be  of 
interest.  The  author  recently  made  some  experiments  with  a  char- 
acteristic steel,  5  ins.  in  diameter,  and  analyzing  0.44  per  cent,  car- 
bon, 0.60  per  cent,  manganese,  and  1.50  per  cent,  silicon.  The 
purpose  was  to  determine  the  possibility  of  using  this  steel  in  place 
of  a  0.60  per  cent,  chrome  steel  with  approximately  the  same  man- 
ganese and  carbon  content.  The  principal  requirement  to  be  met 
was  to  obtain  a  glass-hard  surface  such  as  could  be  obtained  with  the 
chrome  steel.  It  was  found  that  the  hardness  requirement  could 
not  be  met  with  the  silico-manganese  steel,  and  more  important, 


352 


STEEL  AND   ITS  HEAT  TREATMENT 


that  the  steel  would  often  split  or  crack  when  quenched  in  either 
the  cold  water  or  brine  bath  used  for  the  chrome  steel. 

EXPERIMENTS  WITH  SILICO-MANGANESE  GEAR  STEELS 


No. 

1 
2 
3 
4 
5 

Chemical  Analysis. 

Annealed  at  1650°  F. 

Car- 
bon. 

Man- 
ganese 

Silicon 

Tensile 
Strength, 
Lbs.  per 
Sq.  In. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elon- 
gation 

% 

Reduce 
tion 
of 
Area. 

% 

Brinell 
Hard- 
ness. 

Shock 
Test. 

0.40 
0.57 
0.50 
0.70 
0.39 

0.57 
0.61 
0.64 
0.78 
0.52 

1.89 
1.22 
1.64 
1.87 
1.98 

105,820 
127,860 
123,450 
142,510 
109,230 

68,410 
74,100 
82,490 
80,790 
78,510 

19 
15 
12.5 
15.5 
20 

40 
29 
21 
35 
43 

207 
225 
215 
241 
203 

36.1 
21.7 
32.5 
23.3 
43.4 

6 

0.45 

0.42 

0.43 

87,760 

48,780 

19.5 

51 

197 

43.4 

No. 

Quenched  oil  from  1520°  F.,  drawn  at  930°  F.* 

Tensile 
Strength, 
Lbs.  per  Sq.  In. 

Elastic  Limit, 
Lbs.  per  Sq.  In. 

Elongation. 

Reduction 
of  Area. 

Brinell 
Hardness. 

Shock 

Test. 

1 
2 
3 
4 
5 

178,380 
219,460 
156,450 
210,500 
135,120 

159,300 
204,500 
136,830 
197,700 
.107,810 

4.5 
5.5 
4 
2.5 
11 

12 
15 
19 
17 
43 

315 
467 
307 
435 
274 

39.8 
25.3 
75.9 
47 
47 

6 

278,770 

271,660 

3.1 

14 

422 

39.8 

*  Nos.  5  and  6  were  quenched  from  1560°  F. ;  No.  6  was  drawn  at  400°  F. 
The  critical  ranges  of  No.  5  were:  Ac,  1560°;  Ar,  1410°.     For  No.  6,  Ac,  1420°- 
Ar,  1290°. 

The  most  important  use  for  straight  silicon  steels  is  that  for 
electromagnets  and  for  other  electrical  purposes  demanding  a  igh 
magnetic  permeability  or  electrical  resistance.  Hadfield's  silicon 
steel,  containing  approximately  2.75  per  cent,  silicon,  and  with  car- 
bon, manganese  and  the  other  impurities  as  low  as  possible,  is  repre- 
sentative of  this  class.  His  treatment  for  this  steel  consists  of  first 
heating  it  to  about  1950°  F.  and  cooling  quickly,  and  then  heating 
to  1380°  F.  and  cooling  very  slowly,  and  which  is  sometimes  fol- 
lowed by  a  reheating  to  1475°  F.  and  cooling  very  slowly. 

Another  silicon  steel,  used  in  place  of  dynamo  sheet  iron,  specifies 
similar  carbon,  manganese,  etc.,  but  with  a  silicon  content  of  about 
3.25  per  cent.  The  thermal  treatment  recommended  for  this  steel 
is  a  thorough  heating  at  about  1430°  to  1475°  F.,  followed  by  very 
slow  cooling. 


MANGANESE,  SILICON  AND  OTHER  ALLOY  STEELS   353 

TUNGSTEN   STEELS 

The  pearlitic  low-tungsten  steels  when  quenched  from  the  proper 
temperature  do  not  appear  to  be  any  more  modified  by  this  quench- 
ing than  are  the  corresponding  straight  carbon  steels;  the  effect  of 
tungsten  in  such  steels  is,  however,  to  increase  the  tensile  strength, 
with  the  degree  of  brittleness  remaining  about  the  same.  For  this 
reason  tungsten  is  sometimes  used  in  place  of  silicon — which  has  a 
feeble  resistance  to  shock — for  springs.  The  following  table  gives 
the  analysis  and  physical  properties  of  a  characteristic  low-tungsten 
spring  steel: 


Carbon 0.45% 

Manganese 0.22% 


Silicon 0.30% 

Tungsten 0.60% 


Annealed. 

Quenched  in  Oil  from 
1560°,  Drawn  at 
930°  F. 

Tensile  strength,  Ibs.  per  sq.  in  
Elastic  limit,  Ibs.  per  sq  in 

113,500-121,000 

85  000 

185,000 
-      128  000 

Elongation,  per  cent  

14 

7 

The  use  of  tungsten  for  ordinary  structural  purposes  is  mainly 
limited  by  the  fact  that  such  steels  have  to  be  made  by  the  crucible 
process. 

The  other,  and  most  important  uses  for  tungsten,  are  those  for 
permanent  magnets  (the  steel  usually  being  used  in  the  hardened 
condition),  and  for  various  varieties  of  tool  steels  in  both  high- 
speed and  water-  or  oil-hardening  types .  Since  these  steels  involve 
such  a  multitude  of  analyses  and  treatments,  and  form  a  subject  of 
their  own,  it  has  been  deemed  best  to  omit  any  further  discussion, 
but  to  refer  the  reader  to  works  already  published. 


MOLYBDENUM   STEELS 

On  account  of  its  high  cost  the  use  of  molybdenum  has  been 
largely  confined  to  high-speed  and  similar  steel — and  even  there  it 
has  usually  been  superseded  by  tungsten.  In  the  lower  percentages, 
molybdenum  may  be  present  in  steel  as  a  fairly  easily  decomposable 
iron-molybdenum  compound;  with  larger  amounts  of  both  molyb- 
denum and  carbon  it  is  generally  believed  that  the  molybdenum 
forms  a  double  carbide  in  a  similar  manner  to  chrome. 

In  the  pearlitic  molybdenum  steels  the  influence  of  molybdenum 
is  much  like  that  of  chrome,  in  that  it  increases  the  tendency  to 


354 


STEEL  AND  ITS  HEAT  TREATMENT 


greater  hardness  with  proper  quenching  or  with  cold  work,  and  like- 
wise to  increased  brittleness  upon  prolonged  heating  at  high  temper- 
atures. On  the  other  hand,  the  molybdenum  steels  have  a  markedly 
higher  ductility  and  toughness,  besides  an  increased  dynamic 
strength.  The  best  results  (disregarding  its  use  for  tools)  have 
been  obtained  with  the  use  of  1  to  2  per  cent,  molybdenum,  in 
combination  with  the  proper  proportion  of  carbon — the  carbon  having 
a  marked  influence  upon  the  physical  properties  of  molybdenum 
steels.  Considerable  experimentation  has  been  carried  out  with 
pearlitic  molybdenum  steels  for  rifle  barrels  and  large  guns;  it  has 
also  been  used  in  high-duty  machine  parts  such  as  propeller-shaft 
forgings.  It  is  reported  that  excellent  results  have  been  obtained 
in  case-hardening  steels  with  about  1  per  cent,  molybdenum. 

The  influence  of  molybdenum  depends  largely  upon  the  heat 
treatment,  as  is  shown  in  the  following  series  of  tests  by  Giessen 
with  steels  containing  1,  2,  4,  and  8  per  cent,  molybdenum: 

1.00  PER  CENT.  MOLYBDENUM  STEEL 


Chemical. 

As  Rolled. 

No. 

C. 

Mo. 

Tensile 
Strength, 

Elastic 
Limit, 

Elongation, 
Per  Cent. 

Reduction 
of  Area 

Lbs.  per  Sq.  In. 

Lbs.  per  Sq.  In. 

in  2  Ins. 

Per  Cent. 

1 

0.195 

1.03 

67,040 

44,800 

33.31 

64.32 

2 

0.445 

1.05 

108,800 

78,800 

19.5 

49.23 

3 

0.87 

1.02 

160,000 

104,000 

14.5 

34.36 

4 

1.215 

1.10 

117,340 



1.0 

2.02 

Annealed 


No. 

Tensile 
Strength, 
Lbs.  per 
Sq.  in. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elon- 
gation 
Per  Cent, 
in  2  Ins. 

Reduc- 
tion of 
Area, 
Per  Cent. 

Bend 
Test, 
Deg. 

Alter- 
nat- 
ing 
Str'gth 

Brinell 
Hard- 
ness. 

Sclero- 
scope 
Hard- 
ness. 

1 

52,300 

27,800 

35.5 

65  75 

180 

336 

99 

11 

2 

71,420 

38,720 

25.0 

39.2 

180 

210 

131 

13 

3 

108,100 

52,000 

17.22 

22.25 

67 

103 

228 

23 

4 

85,300 

52,100 

5.55 

7.5 

25 

14 

207 

22 

Heat  Treated  (Hardened  in  oil,  reheated  to  1025°  F.). 


1 

90,100 

47,150 

27.46 

68.4 

180 

301 

241 

27 

2 

210,560 

168,600 

14.08 

49.2 

180 

137 

387 

37 

3 

240,490 

193,700 

9.15 

25.2 

16 

92 

418 

44 

4 

279,000 

203,900 

4.92 

12.0 

34 

71 

512 

45 

MANGANESE,  SILICON  AND  OTHER  ALLOY  STEELS  355 
2.00  PER  CENT.  MOLYBDENUM  STEEL 


Chemical. 

As  Rolled. 

No. 

Tensile 

» 
Elastic 

Elongation, 

Reduction 

C. 

Mo. 

Strength, 

Limit, 

Per  Cent. 

of  Area, 

Lbs.  per  Sq.  In. 

Lbs  perSq.  In. 

in  2  Ins. 

Per  Cent. 

5 

0.246 

2.176 

117,820 

21.05 

57.0 

6 

0.442 

2.181 

150,980 

16.7 

46.41 

7 

0.883 

2.186 

198,910 

124,250 

12.1 

32.07 

8 

1.21 

2.109 

216,830 

169,340 

7.04 

9.6 

Annealed 


No. 

Tensile 
Strength, 
Lbs.  per 
Sq.  In. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elon- 
gation 
Per  Cent, 
in  2  Ins. 

Reduc- 
tion of 
of  Area, 
Per  Cent. 

Bend 
Test, 
Deg. 

Alter- 
nat- 
ing 
Str'gth 

Brinell 
Hard- 
ness. 

Sclero- 
scope 
Hard- 
ness. 

5 

65,070 

31,580 

33.3 

62.5 

180 

370 

116 

15 

6 

82,300 

43,400 

27.7 

44.3 

180 

259 

143 

18 

7 

107,070 

54,770 

18.8 

27.5 

100 

126 

207 

22 

8 

95,200 

61,710 

9.4 

13.5 

43 

27 

196 

22 

Heat  Treated  (Hardened  in  oil,  reheated  to  1025°  F.). 


5 

171,140 

115,400 

15.49 

54.4 

180 

172 

387 

35 

6 

211,460 

149,100 

14.08 

47.2 

180 

103 

444 

39 

7 

260,800 

178,800 

5.63 

12.0 

26 

80 

512 

47 

g 

270,940 

16 

39 

512 

48 

4.00  PER  CENT.  MOLYBDENUM  STEEL 


No. 

Chemical. 

As  Rolled. 

C. 

Mo. 

Tensile 
Strength, 
Lbs.  per  Sq.  In. 

Elastic 
Limit, 
Lbs.  per  Sq.  In. 

Elongation, 
Per  Cent, 
in  2  Ins. 

Reduction 
of  Area, 
Per  Cent. 

9 
10 
11 
12 

0.19 
0.487 
0.865 
1.06 

4.11 
4.01 
4.00 
4.02 

119,120 
188,160 
230,270 
239,230 

75,370 
120,060 

179,900 

21.70 
13.5 
8.0 
10.56 

52.71 
33.81 
17.27 
18.40 

356 


STEEL  AND   ITS  HEAT  TREATMENT 

Annealed 


No. 

Tensile 
Strength, 
Lbs.  per 
Sq.  In. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elon- 
gation, 
Per  Cent, 
in  2  Ins. 

Reduc- 
tion of 
Area, 
Per  Cent. 

Bend 
Test, 
Deg. 

Alter- 
nat- 
ing 
Str'gth 

Brinell 
Hard- 
ness. 

Sclero- 
scope 
Hard- 
ness. 

9 

63,390 

31,470 

42.7 

72.5 

180 

366 

116 

17 

10 

77,060 

42,220 

28.3 

52.0 

180 

247 

143 

18 

11 

94,300 

45,920 

20.5 

34.0 

180 

146 

179 

20 

12 

92,960 

42,560 

15.5 

20.5 

94 

66 

196 

23 

Heat  Treated  (Hardened  in  oil,  reheated  to  1025°  F.) 


9 

88,180 

66,500 

30.20 

64.0 

180 

329 

286 

28 

10 

188,900 

139,780 

11.26 

41.6 

123 

109 

444 

43 

11 

258,050 

203,940 

4.22 

4.8 

56 

52 

512 

44 

12 

282,240 

267,000 

7.04 

23.2 

4 

45 

532 

48 

8.00  PER  CENT.  MOLYBDENUM  STEEL 

No. 

Chemical. 

As  Rolled. 

C. 

Mo. 

Tensile 
Strength, 
Lbs.  per  Sq.  In. 

Elastic 
Limit, 
Lbs.  per  Sq.  In. 

Elongation, 
Per  Cent, 
in  2  Ins. 

Reduction 
of  Area, 
Per  Cent. 

13 
14 
15 
16 

17 

0.135 
0.361 
0.445 
0.775 
1.125 

8.01 

8.17 
8.11 
7.85 
7.92 

92,290 
148,290 
215,040 
193,890 
245,500 

154,110 
149,090 
189,950 

25.7 
19.4 
19.71 
9.85 
8.45 

52.22 
45.9 
34.0 
18.4 
16.4 

Annealed 


No. 

Tensile 
Strength, 
Lbs.  j>er 
Sq.  In. 

Elastic 
Limit, 
Lbs.  per 
Sq.  In. 

Elon- 
gation, 
Per  Cent 
in  2  Ins. 

Reduc- 
tion of 
Area, 
Per  Cent. 

Bend 
Test, 
Deg. 

Alter- 
nat- 
ing 
Str'gth 

Brinell 
Hard- 
ness. 

Sclero- 
scope 
Hard- 
ness. 

13 

79,070 

41,660 

31.1 

58.75 

180 

283 

143 

16 

14 

77,060 

34,720 

36.6 

68.23 

180 

273 

143 

18 

15 

83,220 

38,640 

32.2 

57.5 

180 

215 

156 

18 

16 

87,580 

45,140 

22.2 

35.5 

171 

108 

170 

20 

17 

92,290 

48,830 

16.1 

24.0 

85 

66 

187 

22 

Heat  Treated  (Hardened  in  oil,  reheated  to  1025°  F.) 


13 

82,080 

53,760 

30.9 

65.6 

180 

239 

163 

15 

14 

105,350 

75,450 

25.3 

54.4 

180 

226 

351 

30 

15 

127,400 

77,800 

21.1 

49.2 

180 

122 

444 

39 

16 

247,300 

216,830 

7.74 

23.2 

34 

33 

512 

42 

17 

4 

24 

512 

46 

CHAPTER  XVI 

TOOL  STEEL  AND  TOOLS 

THE  problem  of  selecting  a  proper  grade  of  steel  in  relation  to 
the  work  required  is  one  hitherto  met  by  the  steel  manufacturer 
alone.  Until  recently  he  has  recommended  this  or  that  steel  for  a 
given  requirement,  depending  more  or  less  upon  his  general  knowl- 
edge of  the  purpose  for  which  the  tool  is  to  be  used,  and  upon  the 
experience  of  his  customers  in  the  past.  But  with  the  entrance  of 
the  technical  man  into  manufacturing  concerns  and  the  great  im- 
provements resulting  therefrom,  a  fuller  knowledge  of  various  steels, 
their  composition,  applicability  and  efficiency  has  been  demanded. 
This  has  resulted  in  a  wider  dissemination  of  information  regarding 
the  physical,  chemical  and  mechanical  properties  of  steels  manu- 
factured by  various  steel  companies,  and  a  corresponding  education 
of  both  maker  and  buyer. 

Grade.— For  the  aid  and  information  of  their  customers,  the  steel 
maker  usually  groups  his  tool-steel  products  into  various  "  grades  " 
and  "  tempers."  The  former  term  refers  to  the  "  quality  "  of  the 
steel,  according  to  the  class  of  raw  material  which  has  been  used, 
together  with  the  skill  and  care  taken  in  producing  the  finished 
material.  The  highest  grades  should  be  used  for  tools  operating 
under  severe  working  conditions,  demanding  great  endurance  and 
resistance  to  torsional  or  other  strains,  or  upon  which  a  large  labor 
cost  has  been  placed.  These  conditions,  such  as  are  found  in  expen- 
sive dies,  milling  cutters,  taps,  etc.,  would  require  a  high-grade 
steel.  For  such  purposes  as  mill-picks,  cheap  tools,  etc.,  it  would 
be  folly  to  use  any  but  a  lower-quality  steel.  Wear,  the  cost  of 
redressing,  regrinding  and  heat  treatment  are  other  factors  which 
must  be  considered  in  the  selection  of  the  proper  and  most  economical 
steel  which  will  give  the  greatest  efficiency  in  all  senses  of  the  word. 
With  this  in  mind,  the  following  brief  synopsis  is  given: 

1.  Finest  tools  and  dies:   expense  for  material  the  smallest  item 
entering  into  the  cost  and  upkeep  of  the  finished  tool ; 

2.  Finishing  tools   for  lathe   and   planer    work;    special    taps, 

357 


358  STEEL  AND   ITS  HEAT  TREATMENT 

reamers,  milling  cutters  and  other  similar  tools  requiring  a  high- 
grade  steel;  wood-working  and  corrugating  tools; 

3.  General  tool  purposes; 

4.  Ordinary  purposes,  such  as  chisels,  smith  and  boiler  shop 
work,  etc. 

5.  For  rough  or  heavy  work. 

Expressing  this  in  a  different  way,  we  may  say  that  the  choice 
of  a  grade  of  tool  steel  depends  upon  three  factors : 

1.  The  precision  of  the  work  required  of  the  tool; 

2.  The  relative  cost  of  the  steel  in  comparison  with  the  labor 
involved  in  the  manufacture  of  the  tool; 

3.  The  life  of  the  finished  tool  and  its  relation  to  the  cost  of  pro- 
duction. 

Temper. — Carbon  tool  steels  are  further  denoted  by  the  "  tem- 
per." In  tool-steel  sales  parlance  this  refers  to  the  percentage  of 
carbon  in  the  steel  and  may  be  denoted  by  figures  or  letters.  Such 
classifications  generally  refer  to  a  10-point  carbon  limit — thus  No.  7 
temper  may  refer  to  0.65  to  0.75  per  cent,  carbon,  or  it  may  be 
represented  by  whatever  the  individual  company  has  arbitrarily 
selected.  In  this  connection  it  should  be  noted  that  this  "  temper  " 
does  not  refer  to,  and  should  not  be  confused  with  the  word  temper 
as  indicating  the  operation  of  "  letting  down  "  the  steel  after 
hardening. 

General  recommendations  for  the  proper  carbon  content  to  use 
for  various  tools  are  given  in  the  following  table;  these,  however, 
must  not  be  regarded  as  absolute,  for  much  will  depend  upon  the 
grade  of  steel  and  upon  the  exact  use  of  the  tool, 


APPROXIMATE  CARBON  CONTENT  FOR  ORDINARY  TOOLS 

Carbon,  rp     i 

Per  Cent. 

1.50    Tools  requiring  extreme  hardness.     For  turning  chilled- 
rolls  and  tempered  gun-forgings.     Roll  corrugating. 
1.40    Hard   lathe   work   generally.     Chilled-roll   turning.     Cor- 
rugating. 
Graver  tools. 
Brass-working  tools. 

1 . 30    General  lathe,  slotter  and  planer  tools. 
Razors. 
Drawing  dies, 


TOOL  STEEL  AND  TOOLS  359 


Mandrels,    granite    points,   scale   pivots,   bush   hammers, 
peen-hammers. 

Ball-races. 

Files. 

Trimming  dies.     Cutting  dies. 
1  .  20    Twist  drills.     Small  taps. 

Screw  dies,  threading  dies. 

Edge  tools  generally.     Cutlery. 

Cold  stamping  dies,  leather-cutting  dies,  cloth  dies,  glove 
dies. 

Nail  dies,  jewelers'  rolls  and  dies. 
1  .  10    Milling  cutters  and  circular  cutters  of  all  descriptions. 

Wood-working  tools,  forming  tools,  saws,  mill  picks,  axes. 

Small  punches. 

Taps. 

Cup  and  cone  steel. 

Small  springs.     Anvils. 
1.00    Reamers,  drifts,  broaches. 

Large  milling  cutters,  saw  swages. 

Springs. 

Mining  drills,  channeling  drills. 

Large  cutting  and  trimming  dies. 
0.90    Hand  chisels,  punches. 

Drop  dies  for  cold  work,  small  shear  knives. 

Chipping  chisels. 

Cutting  and  blanking  punches  and  dies. 
0.80    Large  shear  knives,  chisels,  hammers,  sledges,  track  chisels. 

Cold  sets,  forging  dies,  hammer  dies,  boiler-maker's  tools. 

Vise-jaws.     Oil-well  bits  and  jars.     Mason's  tools. 
0.  70    Smith  shop  tools,  track  tools,  cupping  tools,  hot  sets. 

Set  screws. 

0.60    Hot  work  and  battering  tools  generally.     Bolt  and  rivet 
headers. 

Hot    drop    forging  dies.     Rivet  sets.     Flatteners,   fullers, 

wedges. 

0.50     Machinery  parts.     Track  bolt  dies  where  water  is  con- 
tinually running  on  dies  (hot  work). 

Navy   Specifications.  —  The   United    States    Navy  specifies   the 
following  straight  carbon  tool  steel  for  its  general  requirements  : 


360 


STEEL  AND   ITS   HEAT   TREATMENT 


Class. 

I. 

II. 

III. 

IV. 

Carbon  
Manganese 

1.25-1.15 
0.35-0.15 

1.15-1.05 
0.35-0.15 

0.95-0.85 
0.35-0.15 

0.85-0.75 
0  35-0  15 

Phosphorus  
Sulphur          .      .  . 

0.015-0 
0.02-0 

0.015-0 
0.02-0 

0.02-0 
0.02-0 

0.02-0 
0  025-0 

Silicon 

0.40-0  10 

0.40-0.10 

0.40-0  10 

0  40-0  10 

Chrome  and  vanadium  optional. 

Class  I.  Lathe  and  planer  tools,  drills,  taps,  reamers,  screw-cutting 

dies;  taps  and  tools  requiring  keen  cutting  edge  combined  with 

great  hardness. 
Class  II.  Milling  cutters,  mandrels,  trimmer  dies,  threading  dies, 

and  general  machine-shop  tools  requiring  keen  cutting  edge 

combined  with  hardness. 
Class  III.  Pneumatic  chisels,  punches,  shear-blades,  etc.,  and  in 

general  tools  requiring  hard  surface  with  considerable  tenacity. 
Class  IV.  Rivet  sets,  hammers,  cupping  tools,  smith  tools,  hot-drop 

forge  dies,  etc.;   tools  requiring  great  toughness  combined  with 

necessary  hardness. 

The  Navy  Department  also  maintains  the  requirements  as  to 
grade  by  requiring  a  steel  which  will  stand  rehardening  a  specified 
number  of  times  without  cracking. 

General  Properties. — The  following  table  shows  the  relative 
toughness  and  hardness  of  tool  steel  of  the  different  carbon  contents : 

Carbon, 
Per  Cent. 

0 . 50    Toughness  only. 

0.60     Great  toughness  with  properties  suitable  for  hardening  and 

tempering. 

0.70  Excellent  toughness,  but  with  cutting  edge. 
0.80  Tough  tool  steel,  withstanding  shocks,  etc. 
0.90  Good  cutting  edge  but  with  toughness  an  important 

factor. 

1 . 00    Toughness  and  cutting  edge  about  equal. 
1 . 20    Great  hardness  combined  with  some  toughness. 
1  30    Great  hardness  in  cutting  edge.     Toughness  slight  factor. 
1.40    Extreme    hardness    in    cutting    edge    first     requirement. 

Toughness  slight  factor. 

Some  metallurgists  consider  that  it  is  safer  to  select  a  too  hard  steel 
and  draw  the  temper  at  a  higher  temperature  than  to  choose  a  too 


TOOL  STEEL  AND   TOOLS 


361 


soft  steel  with  a  view  to  increasing  its  hardness  by  a  weaker  temper- 
ing. Opposed  to  this  is  the  fact  that  the  higher  the  carbon 
content  the  more  the  care  which  will  be  required  in  the  harden- 
ing operation,  since  the  steel  becomes  more  sensitive  to 
overheating. 


GENERAL  TEMPERING  COLORS  FOR  TOOLS 

Faint  yellow:     Steel-engraving  tools. 

Light  turning  tools. 

Hammer  faces. 

Planing  tools  for  steel. 

Ivory-cutting  tools. 

Planing  tools  for  iron. 

Paper-cutting  knives. 

Wood-engraving  tools. 
Light  yellow:     Milling  and  other  circular  cutters  for  metal. 

Bone-cutting  tools. 

Scrapers  for  brass. 

Shear  blades  in  general. 

Boring  cutters. 

Leather-cutting  dies. 

Screw  dies. 

Inserted  saw  teeth. 

Taps. 

Rock  drills. 

Chasing  tools. 

Penknives. 
Straw:  Dies  and  punches  in  general. 

Moulding  and  planing  cutters  for  hardwood. 

Reamers. 

Gouges. 

Brace  bits. 

Plane  irons. 

Stone-cutting  tools. 
Deep  straw:      Twist  drills. 

Cup  tools. 

Wood  borers. 

Circular  saws  for  cold  metal. 

Cooper's  tools. 

Augers. 


362 


STEEL  AND   ITS  HEAT  TREATMENT 


Brown.  Drifts. 

Circular  cutters  for  wood. 

Dental  and  surgical  instruments. 

Axes  and  adzes. 

Saws  for  bone  and  ivory. 
Peacock:  Cold  sets  for  steel  and  cast  iron. 

Hand  chisels  for  steel  and  iron. 

Boiler-maker's  tools. 

Firmer  chisels. 

Hack  saws. 
Purple.  Moulding  and  planer  cutters  for  soft  wood. 

Smith  tools  and  battering  tools  generally. 
Blue :  Screwdrivers. 

Saws  for  wood. 

Springs  in  general. 

These  colors  are  for  general  crucible  steel  with  low  manganese. 
Their  applicability  to  particular  work  and  special  steels  may  be  taken 


1460 


1440 


•51420 


>1400 


1380 


1360 


Diameter  in  Inches 
FIG.  199. — Temperature-size  Curve  for  Hardening  Tools. 


in  a  general  way,  but  that  temperature  must  be  adopted  which  will 
suit  the  special  work  or  steel  in  hand. 

Hardening. — We  have  previously  discussed  the  fact  that  with  any 
increase  in  the  mass  of  the  steel  there  is  a  corresponding  decrease 
in  both  the  maximum  surface  hardness  and  the  depth  of  hardness, 
when  quenched  from  the  same  temperature.  This  difference  in 
hardness  is  due  to  the  difference  in  the  rate  of  cooling  of  the  small 


TOOL  STEEL  AND  TOOLS 


363 


and  large  sections.  In  order  to  produce  the  same  degree  of  hard- 
ness in  a  small  and  large  section,  as  applied  to  small  tools,  it  is  neces- 
sary to  heat  the  larger  section  hotter  for  hardening  than  the  smaller. 
To  illustrate:  Matthews  and  Stagg  have  worked  out  the  relation  of 
mass  to  temperature  for  one  particular  grade  of  the  same  tool  steel 
in  which  the  sizes  varied  from  Y&  m-  diameter  to  f  in.  diameter,  and 


1200  1300  1400  1500  1600 

FIG.  200. — Loss  of  Hardness  Due  to  High  Hardening 


1700  1800 

Tempera  tures .     (Shore . ) 


found  that  a  difference  of  about  60°  F.  in  heating  was  necessary  to 
produce  the  same  degree  of  hardness  in  the  two  extreme  sizes.  Their 
temperature-size  curve  is  given  in  Fig.  199. 

The  table  on  page  365  gives  the  approximate  temperatures  for 
handling  general  tool  steels.  Two  columns  are  given  under  harden- 
ing temperatures  as  representing  the  best  practice  of  two  well-known 


364  STEEL  AND   ITS  HEAT  TREATMENT 

steel  companies.  As  a  general  proposition,  the  lowest  temperature 
should  be  used  for  hardening  which  will  give  the  desired  results: 
the  use  of  abnormally  high  temperatures  will  increase  the  grain  size, 
weaken  the  steel,  and  reduce  the  hardness.  These  last  factors 
become  even  the  more  apparent  with  increase  in  the  carbon  content, 
as  is  roughly  illustrated  by  the  scleroscope  readings  as  given  in 
Shore's  chart  in  Fig.  200. 

On  the  other  hand,  on  account  of  mass  action  and  other  individual 
and  distinctive  shop  conditions,  it  is  difficult  to  set  the  upper  limit 
over  which  hardening  should  not  be  done.  Certain  classes  of  work 
often  require  temperatures  which  might  prove  excessive  for  other 
work;  thus  one  instance  has  come  to  the  author's  attention  in  which 
the  hardening  of  certain  1  -^-in.  rounds  of  0.9  per  cent,  carbon  stock 
are  hardened  at  1600°  to  1620°  F.  and  80  per  cent,  more  service  is 
being  obtained  than  from  the  same  steel  hardened  at  1460°  F. 
Again,  another  well-known  company  hardens  0.9  per  cent,  carbon 
steel  of  approximately  the  same  size  at  1370°  F.  and  obtains  better 
service  than  when  hardened  at  higher  temperatures.  Each  case, 
in  other  words,  must  be  handled  separately  and  those  temperatures 
worked  out  which  will  give  the  best  solution  of  that  particular 
problem. 

Distortion  Factors. — Slender  pieces  of  steel,  when  hot,  will  bend 
under  the  application  of  a  steady,  even  though  slight,  load.  The 
weight  of  the  part  being  heated  for  hardening  is  often  sufficient  to 
cause  noticeable  distortion  if  the  tool  is  placed  in  the  furnace  in 
such  a  manner  that  it  is  not  carefully  supported.  For  this  reason, 
such  tools  are  best  heated  when  held  in  a  vertical  position,  with  the 
point  of  support  at  the  upper  end  of  the  piece,  the  tool  being  so  held 
that  it  automatically  comes  to  the  normal  position  as  will  a  plumb- 
bob. 

Distortion  may  be  due  to  the  initial  condition  of  the  steel,  such 
as  may  result  from  forging,  rolling,  machining,  etc.  Any  strains 
which  exist  in  the  tool  previous  to  heating  for  hardening  are  relieved 
when  the  piece  is  heated,  but  the  readjustment  of  such  strains  may 
cause  a  bending  or  twisting  of  the  tool.  In  making  the  tool  it  is 
advisable  to  rough  down  to  within  about  ^  in.  of  the  finished  size 
and  then  anneal  in  some  non-oxidizing  material  to  relieve  the 
machining  strains.  If  the  tools  are  not  straight  after  annealing, 
they  should  be  heated,  straightened  while  hot  (do  not  straighten  in 
the  cold),  and  then  reannealed.  The  tools  are  then  finished  and 
are  ready  for  hardening. 


TOOL   STEEL  AND  TOOLS 


365 


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366  STEEL  AND   ITS  HEAT  TREATMENT 

The  influence  of  annealing  or  previous  hardening  operations 
upon  the  change  in  shape,  such  as  increase  or  decrease  in  diameter 
and  length,  or  combinations  of  these,  is  very  difficult  to  foretell. 
As  a  general  rule,  however,  such  changes  are  the  more  marked  with 
repetitive  hardenings,  and  with  increased  percentages  of  carbon. 

Changes  in  Length. — Most  tool  steel  has  a  tendency  to  contract 
in  length  upon  hardening,  and  especially  after  previous  annealing 
or  hardenings.  This  change,  whether  it  is  expansion  or  shrinkage,  is 
dependent  upon  the  chemical  composition  and  uniformity  of  the 
steel,  the  grain  size  and  influence  of  the  mechanical  elaboration 
and  annealing,  the  uniformity  and  amount  of  heat  used  in  hardening, 
the  method  and  rapidity  of  cooling,  and  innumerable  other  variables. 
The  commercial  application  of  heat-treatment  principles  when  ap- 
plied to  fine  tools  for  their  standardization  to  exact  measurements 
must  be  along  the  lines  of  standardizing  each  step  of  the  process. 
The  steel  must  initially  be  kept  uniform  in  composition  and  physical 
test;  the  limits  of  treatment  must  be  held  to  a  minimum  difference: 
and  having  somewhat  accomplished  these  aims,  the  average  change 
of  size  for  specific  dimensions  must  be  studied  and  the  tool  made 
accordingly.  Thus  in  the  matter  of  taps,  by  obtaining  the  average 
contraction  lengthwise  for  a  given  size  tap  blank,  under  standard 
conditions,  the  thread  may  be  cut  upon  lathes  having  the  lead  screws 
so  adjusted  that  the  pitch  given  to  the  tap  before  hardening  will 
just  come  right  after  hardening. 

Changes  in  Diameter. — Assuming  that  the  many  other  variables 
affecting  distortion  might  be  reduced  to  a  constant  or  standard,  it 
is  a  generally  accepted  condition  that  there  is  a  relationship  existing 
between  the  amount  of  distortion  by  swelling  after  hardening  and 
the  original  diameter  of  the  piece.  Most  slender  tools,  in  ordinary 
commercial  practice,  have  a  tendency  to  expand  in  diameter  after 
hardening.  In  factories  where  a  standard  steel  and  standard  methods 
are  in  vogue  the  amount  of  increase  is  a  very  important  factor,  as 
is  shown  by  the  following  data  obtained  from  an  exhaustive  study  1 
of  taps: 

Thus  this  particular  problem  was  attacked  with  the  view  of 
obtaining,  under  standard  shop  conditions,  the  average  increase  in 
diameter  due  to  hardening  for  each  size  of  tap.  With  these  results 
it  was  then  possible  to  determine  the  necessary  angle  diameter  of 
the  tap  before  hardening,  so  that  the  hardened  tap  would  meet  the 
final  requirements. 

i  Woodward. 


TOOL  STEEL  AND  TOOLS 


367 


Diameter  of 
Tap. 

Average  Increase 
in  Diameter. 

Diameter  of 
Tap. 

Average  Increase 
in  Diameter. 

yjj  inch 

1^  inch 

0  0025 

0.00025 

0.0025 

i 

0.0005 
0.001 

2 

0.003 
0.003 

1 

i 

0.0015 
0.002 

3 

0.0035 
O.Q035 

u 

0.002 

4 

0.004 

It  was  also  found,  however,  that  experiments  upon  the  same  steel, 
under  apparently  the  same  conditions,  showed  that  there  may  be 
very  great  variations  in  the  effect  of  hardening  upon  the  diameter; 
and  when  the  various  other  factors  are  taken  into  account,  the 
difficulty  of  prognosticating  exactly  what  is  going  to  happen  is  even 
more  apparent. 

Heating.— Much  of  the  difficulty  experienced  through  distortion 
or  cracking  may  be  largely  diminished  by  the  proper  application  of 
the  principles  of  heating  such  as  have  been  discussed  elsewhere. 
The  heating  should  be  done  slowly,  carefully  and  at  a  uniform  rate. 
In  no  case  should  the  temperature  of  the  furnace  be  greater  than 
the  maximum  temperature  to  which  the  steel  is  to  be  heated.  After 
the  steel  has  been  thoroughly  heated,  a  further  continuance  will 
only  tend  to  weaken  the  steel  by  increasing  the  grain  size.  The 
furnace  should  be  of  such  design,  construction  and  operation  that 
it  shall  be  of  uniform  temperature  over  its  whole  hearth,  shall  heat 
at  a  uniform  rate,  shall  not  be  greatly  affected  by  the  introduction 
of  fresh  charges,  shall  have  a  neutral  or  reducing  atmosphere,  and 
shall  be  under  exact  control.  Further,  it  is  not  only  the  temperature 
to  which  the  steel  has  been  heated  in  the  furnace  that  counts,  but 
also  the  temperature  (and  uniformity  of  temperature)  of  the  steel 
when  it  goes  into  the  quenching  bath, 


HEAT   TREATMENT    OF    TOOLS 

In  the  following  pages  there  is  given  a  description  of  practical 
methods  of  treating  certain  tools.  It  is  not  intended  that  the  data 
shall  be  comprehensive  of  all  methods  or  of  all  tools,  but  shall  give 
practical  hints  and  methods  which  may  aid  the  man  in  the  small 
shop  who  may  have  such  work  to  do  but  occasionally.  It  should 
also  be  remembered  that  the  ideas  given  are  representative  of  a 


368  STEEL  AND   ITS  HEAT  TREATMENT 

type  of  treatment  or  tool,  and  which  may  find  application  for  many 
uses  not  given.  The  methods  given  are  taken  from  practical  work, 
and  have  given  satisfaction. 

Chisels. — Chisels  belong  to  that  class  of  tools  which  can  be 
advantageously  hardened  without  first  grinding  away  the  "  skin." 
Chisels  are  not  hardened  by  heating  up  the  whole  tool,  but  only 
applying  the  heat  to  2  ins.  or  so  of  the  cutting  end;  too  short  a 
distance  is  detrimental,  as  the  long,  unhardened  shank  may  bend 
under  heavy  blows.  In  quenching,  care  should  be  used  to  avoid  a 
distinct  line  of  demarkation  between  the  hardened  £nd  unhardened 
parts,  as  otherwise  the  end  may  break  off  in  service.  After  harden- 
ing the  cutting  end  by  immersing  in  water  and  at  the  same  time  mov- 
ing the  chisel  up  and  down,  the  tool  should  be  removed  while  suffi- 
cient heat  for  tempering  still  remains  in  the  shank.  The  cutting 
edge  is  brightened  with  emery  cloth.  Temper  colors  will  soon 
appear  in  the  brightened  spot,  which  are  caused  by  the  heat  from 
the  shank  "  running  down  "  towards  the  cutting  edge;  the  temper 
colors  may  be  "  spread  out  "  by  holding  the  end  near  the  fire  if  it 
is  desired  to  have  a  broad  band  near  the  edge.  When  the  proper 
temper  color  is  obtained  at  and  near  the  edge,  the  chisel  should 
be  immediately  plunged  into  water  until  cold  in  order  to  prevent 
further  softening. 

The  degree  of  tempering  depends  upon  the  steel  which  has  been 
used,  the  duties  in  service  and  upon  the  general  experience  and  judg- 
ment of  the  hardener.  In  general  it  may  be  said  that  chisels  for 
metal  should  be  tempered  to  about  a  peacock  color;  for  stone  to  a 
purple  color,  although  many  stone  and  granite  chisels  are  drawn  to 
only  a  light  straw  color;  and  for  soft  material  to  a  blue  color.  If 
a  very  tough  steel  is  desired,  the  chisels  may  be  given  a  double 
tempering  by  heating  to  the  temper  color  twice,  the  first  color 
being  rubbed  off  after  quenching.  The  lower  the  carbon  used  the 
lower  should  be  the  tempering;  in  fact,  a  0.45  per  cent,  carbon  steel 
is  sometimes  used  after  hardening  without  subsequent  tempering. 

An  excellent  plan  in  treating  chisels  is  to  oil  treat  the  blank 
before  forging  out,  quenching  the  steel  in  oil  and  from  a  temperature 
sufficient  to  harden  it  well.  This  will  stiffen  the  shank  and  keep  the 
head  from  upsetting  during  use. 

The  problem  of  producing  a  good  chipping  chisel  is  simple, 
and  yet  has  met  with  more  difficulties  than  the  majority  of  other 
such  tools.  The  correct  adjustment  of  temperatures  plays  a  most 
important  part  in  this  work.  For  forging,  the  steel  should  be  heated 


TOOL  STEEL  AND   TOOLS  369 

up  gradually  and  not  much  beyond  a  bright-red  heat.1  The  work 
should  be  done  rapidly,  aiming  to  obtain  the  greatest  reduction  to 
size  while  the  steel  is  yet  near  its  first  heat;  as  the  temperature  falls, 
the  blows  should  be  lighter  and  quicker.  It  has  been  the  author's 
experience  that  added  toughness  is  given  to  the  steel  if  these  light, 
quick  blows  are  carried  on  until  the  steel  neare  a  dull-red  heat.  If 
reheating  is  necessary,  adopt  the  same  care  in  raising  the  temper- 
ature as  before.  The  chisel  should  be  immediately  hardened  and 
tempered  after  forging  when  possible.  If  the  pressure  of  work 
will  not  permit  of  this,  the  chisel  should  be  allowed  to  cool  as  slowly 
as  possible  by  sticking  the  hot  end  in  dry  dirt;  this  latter  will  tend  to 
eliminate  the  cooling  strains  which  might  otherwise  be  set  up.  For 
hardening  heat  slowly  to  the  lowest  temperature  at  which  the  steel 
will  harden  (generally  about  1350°  to  1400°  F.),  allow  the  heat  to 
penetrate  and  harden  as  usual.  If  care  is  used,  and  sufficient 
thought  has  been  given  to  the  design  of  the  chisel,  a  hundred  per 
cent,  improvement  may  be  easily  obtained  over  the  ordinary  "  hit 
or  miss  "  method. 

An  open-hearth  steel  which  has  given  unusual  service  for  pneu- 
matic chipping  chisels  is  as  follows:  Carbon,  0.90  to  1.00  per  cent.; 
manganese  .50;  phosphorus  and  sulphur  low;  chrome,  0.50  per 
cent.  The  hardening  temperature  of  this  steel  is  about  1400°  F. 

Die  Blocks. — The  problem  of  satisfactorily  treating  die  blocks  is 
one  which  will  try  the  hardener's  knowledge  and  patience,  increas- 
ing in  difficulty  with  the  intricacy  of  design  and  size.  Primarily 
the  requirements  of  any  die  are  (1)  a  hard  face,  (2)  sufficient  depth  of 
hardening  to  prevent  the  impression  from  sinking,  and  (3)  a  tough 
back  or  body  to  take  up  the  shock  or  blow.  Further,  the  hardening 
must  be  conducted  in  such  a  manner  as  will  prevent  any  change  of 
size — such  as  warping — besides  producing  a  clean,  sharp  impression 
free  from  scale,  pitting  or  checks.  These  fundamental  requirements, 
assuming  that  the  right  kind  of  steel  is  used,  involve  the  factors  of 
proper  heating  and  the  most  suitable  method  of  cooling. 

1  For  the  convenience  of  the  tool  hardener  who  has  to  work  without  a  pyrom- 
eter and  must  gauge  temperatures  by  the  eye,  the  following  table  of  approxi 
mate  heat  colors  in  moderate  diffused  daylight  is  given : 

White 2200°  F.  Cherry  or  full  red 1375°  F. 

Light;  yellow 1975  Medium  cherry 1250 

Lemon 1825  Dark  cherry 1175 

Orange 1725  Blood  red 1050 

Salmon 1650  Faint  red 900 

Bright  red 1550 


370  STEEL  AND  ITS  HEAT  TREATMENT 

More  die  blocks  are  warped  or  cracked  through  improper  heat- 
ing than  through  any  other  cause.  It  is  absolutely  essential  that 
the  entire  mass  of  the  steel  of  the  block  shall  be  heated  carefully, 
uniformly,  through  and  through,  to  the  proper  temperature. 

It  is  always  best  to  allow  the  die  to  heat  up  with  the  furnace  so 
that  any  strains  which  may  exist  in  the  steel  may  be  removed  grad- 
ually and  give  the  mass  of  steel  ample  time  to  adjust  itself  to  the  rise 
in  temperature.  If  the  furnace  for  heating  for  hardening  is  already 
at  or  near  the  quenching  temperature  when  the  die  is  ready  for 
heating,  it  will  be  advisable  carefully  to  preheat  the  die  in  a  separate 
furnace.  Equipment  is  subsequently  described  for  automatically 
preheating  and  full-heating  in  the  same  furnace.  In  no  case  should 
the  temperature  of  the  heating  furnace  be  greater  than  the  maximum 
temperature  from  which  the  steel  is  to  be  quenched.  The  practice 
—unfortunately  common — of  forcing  the  furnace  in  order  more 
quickly  to  heat  the  steel  is  to  be  strongly  condemned ;  such  practice 
must  inevitably  result  in  the  overheating  of  the  edges  or  corners 
of  the  die  and  produce  unsatisfactory  results.  The  extra  time 
spent  in  careful  and  uniform  preheating,  and  in  all  subsequent  heat- 
ing operations,  will  be  well  worth  the  expense. 

The  exact  temperature  best  suited  for  hardening  may  be  men- 
tioned here  only  in  a  general  way.  The  chemical  composition  of  the 
steel,  the  size  of  the  die  block,  the  depth  of  hardness  required,  the 
condition  of  the  steel  before  hardening,  and  many  other  factors 
must  be  taken  into  consideration.  In  general,  nickel  and  chrome 
nickel  steels  may  be  quenched  at  lower  temperatures  than  those  used 
for  the  corresponding  carbon  steels,  while  vanadium  and  chrome 
vanadium  steels  usually  require  higher  temperatures.  Again,  the 
greater  the  mass  of  the  steel,  and  the  greater  the  depth  of  hardness 
required,  the  higher  is  the  temperature  for  quenching.  Some  die 
hardeners  even  find  that  a  temperature  which  will  produce  a  slightly 
coarse  grain  is  advisable  for  certain  classes  of  work,  such  as  dies 
for  cold  forming  under  a  heavy  drop.  In  other  words,  the  proper 
temperature  for  hardening  must  be  determined  by  experiment,  but 
the  lowest  temperature  which  will  produce  the  desired  results  is 
always  the  best. 

The  next,  and  a  vastly  important  factor,  is  the  duration  of  heat- 
ing at  the  predetermined  temperature  for  hardening.  The  point 
to  be  emphasized  is  that  the  mass  of  the  steel  must  be  uniformly 
heated  throughout — and  this  takes  time.  Not  only  must  the  outer 
sections  attain  the  necessary  degree  of  heat,  but  also  the  very 


TOOL   STEEL  AND   TOOLS  371 

center  of  the  die.  Disregard  of  this  fundamental  principle  is  the 
basis  of  a  large  proportion  of  the  failures  through  cracking  or  warp- 
ing, and  which  are  so  often  attributed  to  "  the  steel  is  no  good." 
If  the  die  block  lays  directly  on  the  hearth  of  the  furnace  (and  this, 
it  might  be  mentioned,  is  not  the  best  practice,  since  any  work  is 
best  heated  when  the  hot  gas  currents  can  circulate  entirely  around 
it),  the  penetration  of  the  heat  may  be  roughly  determined  by 
moving  the  block  to  one  side  and  noting  the  color  of  the  space  va- 
cated; if  this  area  is  not  of  the  same  color  as  the  rest  of  the  furnace 
floor  the  heat  has  not  thoroughly  penetrated  to  the  center  of  the 
die.  This  test,  called  the  "  drawrouagh  "  by  old  English  hardeners, 
should  be  followed  even  though  the  play  of  heat  colors  over  the  ex- 
posed portions  of  the  block  appear  uniform. 

For  protection  of  the  impression  from  oxidation  through  contact 
with  the  air,  the  faces  of  the  dies  may  be  packed  in  carbonaceous 
material.  One  of  the  large  manufacturers  of  silverware  packs  his 
dies  as  follows:  A  small  sheet-iron  pan,  about  2  ins.  high  and 
about  an  inch  or  so  wider  than  the  die  all  around,  is  partly  filled 
with  granulated  animal  charcoal  or  bone.  The  die  is  then  pressed 
firmly  upon  the  charcoal,  forming  an  impression,  and  is  then  care- 
fully removed.  This  impression  is  sprinkled  with  powdered  animal 
charcoal,  and  with  very  fine  steel  filings.  The  die  is  carefully 
replaced  in  position,  surrounded  with  more  granulated  animal  char- 
coal to  the  height  of  the  pan,  and  the  space  between  the  top  of  the 
pan  and  the  die  carefully  luted  with  fire-clay.  Upon  heating,  the 
filings  and  powdered  charcoal  fuse  together  upon  the  surface  of  the 
steel,  forming  a  protective  coating  which  eliminates  oxidation  during 
heating,  but  which  is  washed  off  during  the  quenching.  A  little 
brushing  with  oil  and  emery  powder  will  immediately  produce  a 
clean,  bright  surface.  Another  method  is  first  to  paint  the  surface 
of  the  die  with  a  thick  paste  made  of  linseed  or  cottonseed  oil  and 
powdered  bone-black;  the  die  is  then  placed  in  a  shallow  pan  upon 
a  half-and-half  mixture  of  fresh  bone  and  powdered  charcoal,  in  a 
suitable  pan  or  box,  which  is  then  filled  and  luted  as  above. 

The  old-time  method  of  an  immediate  and  total  quenching  of 
the  block  until  it  is  quite  cold  should  be  attempted  only  with  the 
simplest  forms  and  small  sizes  of  dies.  Large  blocks  have  a  great 
tendency  to  warp,  bulge,  or  even  crack  if  a  total  immersion  is  adopted, 
this  being  caused  by  the  unequal  contraction  of  the  metal  of  the 
surface  and  of  the  core.  It  may  be  said,  however,  that  this  difficulty 
may  be  largely  avoided  if  the  block  is  previously  given  a  special 


372  STEEL  AND   ITS  HEAT  TREATMENT 

treatment  consisting  of  oil  quenching  from  just  over  the  critical 
range,  followed  by  an  annealing  at  a  temperature  just  under  the 
lower  critical  range.  Total  quenching  also  should  not  be  used  if  an 
extremely  hard  face  is  desired,  since  the  heat  cannot  usually  be 
removed  quickly  enough. 

The  best  practice  for  hardening  large  die  blocks  consists  in  first 
carefully  preheating  the  die,  then  slowly  raising  it  to  the  hardening 
temperature  and  allowing  it  to  soak  at  this  temperature  until  it  is 
thoroughly  heated.  For  this  work  a  furnace  should  be  used  in  which 
accurate  temperatures  and  uniform  heating  can  be  obtained.  Large 
blocks  may  be  most  easily  handled  by  the  use  of  a  hoist  on  a  swinging 
run-way,  mono-rail  or  overhead  crane,  and  equipped  with  tongs 
or  "dogs."  The  dogs  fit  into  holes  which  have  previously  been 
drilled  in  opposite  sides  of  the  block  about  half  way  between  the 
upper  and  lower  faces. 

When  the  block  is  properly  heated,  it  is  removed  to  the  front  of 
the  furnace,  gripped  with  the  dogs,  run  over  to  a  position  above  the 
quenching  tank,  lowered  face-downwards  entirely  into  the  water  or 
oil  for  a  few  seconds  (to  prevent  warping),  and  then  raised  out  of 
the  quenching  bath  until  immersed  about  1  in.  deeper  than  the 
depth  of  the  deepest  impression  in  the  die.  The  surface  of  the  bath 
should  be  kept  in  motion,  or  else  the  block  should  be  slowly  raised 
and  lowered  a  little  so  that  there  will  be  no  one  line  of  hardening. 
The  hardening  will  be  greatly  increased  if  a  stream  or  heavy  spray 
of  water  (assuming  water  to  be  used  for  hardening)  is  directed 
against  the  face  of  the  block,  or  into  the  impression.  In  the  case  of 
blocks  which  contain  a  deep  impression,  such  as  are  used  for  certain 
classes  of  gears,  etc.,  it  will  be  necessary  to  have  a  stream  of  water 
thus  impinge  upon  the  impression  in  order  to  harden  it;  the  face 
of  the  block  will  take  on  great  hardness,  and  the  heat  from  the 
unsubmerged  part  will  gradually  be  drawn  out. 

When  the  face  of  the  block  is  entirely  cold,  and  the  majority 
of  the  heat  taken  out  of  the  other  portion  of  the  block  (usually  at 
about  a  very  dark  red,  but  dependent  upon  the  size  of  the  block), 
it  is  raised  out  of  the  water,  reversed  to  face  up,  and  brightened  with 
emery  paper.  The  heat  in  the  hot  part  of  the  block  will  gradually 
temper  the  hardened  face.  When  this  approaches  a  good  straw  color 
the  block  is  immersed  in  water  or  oil  until  cold;  in  some  instances 
where  a  softer  block  is  desired,  the  block  may  be  allowed  to  cool  in 
the  open  without  the  use  of  water  to  stop  the  temper.  In  case  there 
is  not  sufficient  heat  left  in  the  block  after  hardening  to  bring  out 


TOOL  STEEL  AND  TOOLS 


373 


the  desired  temper  color,  the  block  may  be  stood  in  front  of  the  fur- 
nace, back  to  the  heat,  or  placed  on  a  hot  bar  of  steel,  or  laid  on  top 
of  a  low  smith  fire,  until  the  proper  temper  is  reached.  Die  blocks 
will  generally  give  more  uniform  service  when  drawn  in  a  tempering 
bath  when  such  is  possible.  In  case  the  block  is  of  intricate  design 
and  requires  very  particular  tempering  in  weak  spots,  this  may  be 
done  by  the  local  application  of  heat  by  means  of  hot  plates,  etc. 


FIG.  201. — Intake  End  of  Special  Furnace  for  Hardening  Forging  Dies. 
("  Machinery.") 

Die  blocks  hardened  and  tempered  as  directed  above  should  pro- 
duce a  strong,  tough  base  and  core,  increasing  in  hardness  as  the 
face  is  approached. 

Semi-automatic  furnaces  for  heating  die  blocks  for  hardening, 
in  which  the  preheating  as  well  as  the  final  heating  are  done  in  the 
same  furnace,  are  illustrated  in  Figs.  201  and  202.  Four  runways, 
filled  with  3-in.  malleable-iron  balls,  extend  throughout  the  length 
of  the  heating  floor  of  the  furnace.  Castings  which  fit  over  the 


374 


STEEL  AND  ITS  HEAT  TREATMENT 


balls  in  two  of  these  run-ways  and  which  are  of  suitable  size  to  carry 
one  of  the  dies,  are  placed  in  position  in  the  end  of  the  furnace  shown 
in  Fig.  201.  When  the  cold  die  is  placed  on  this  casting,  as  shown  at 
0,  one  of  the  pneumatic  pushers  P  is  brought  into  play  and  the  cast- 
ings act  as  a  cart,  carrying  the  die  into  the  furnace.  By  following 
the  first  casting  and  its  die  with  others,  the  furnace  is  gradually 
filled,  the  ram  pushing  the  whole  line  of  dies  further  into  the  furnace 
with  each  new  addition.  The  furnace  is  so  designed  and  operated 
that  the  temperature  at  the  charging  end  is  low,  but  gradually 


FIG.  202.— Quenching  and  Tempering  Dies.     ("  Machinery.") 

increases  up  to  the  maximum  near  the  other  end  of  the  furnace; 
preheating  and  the  final  heating  are  thus  obtained  in  the  same 
furnace.  Each  furnace  is  double  tracked  and  heats  two  rows  of 
dies  at  once.  At  the  end  of  the  furnace  shown  in  Fig.  202  the 
hot  dies  come  out  on  the  extension  of  the  run-way  marked  Q.  The 
faces  of  the  dies  are  turned  downwards  so  that  the  dies  may  be 
picked  up  by  the  traveling  crane  and  lowered  into  the  quenching 
tank,  as  shown  at  •  R.  A  stream  of  water  also  plays  against  the 
impression,  as  usual.  The  hot  plate  shown  at  T  is  used  for  the 
tempering. 


TOOL  STEEL  AND  TOOLS 


375 


Engraved  dies  for  spoons,  forks,  knives,  etc.,  are  treated  at  one 
plant  by  the  following  method.  After  packing  and  heating  as 
described  in  a  previous  section,  the  dies  are  quenched  face  up  in 
water  at  a  temperature  of  about  70°  to  80°  F.,  to  a  depth  of  within 
about  \  in.  of  the  face.  Water  at  this  temperature  seems  to  give  the 
best  results  in  this  particular  instance — colder  water  is  too  harsh, 
while  warmer  water  does  not  sufficiently  distribute  the  strains  nor 
give  sufficient  hardness.  As  soon  as  the  cooling  effect  just  begins 
to  creep  towards  the  face  of  the  die,  and  which  only  takes  a  few 


FIG.  203. — Method  of  Hardening  Engraved  Die. 

seconds;  the  die  is  immediately  wholly  immersed  in  a  vertical  posi- 
tion in  the  water,  with  the  impression  turned  toward  a  heavy  stream 
of  water  which  impinges  directly  upon  it.  The  arrangement  of  the 
quenching  bath  is  shown  in  Fig.  203:  the  die  (a)  rests  upon  a  wire 
platform  (6);  the  water  is  supplied  under  pressure  through  a  IJ-in. 
pipe  (c),  flowing  out  through  a  J-in.  slot  (d)  which  extends  from  the 
level  of  the  die  support  to  the  top  of  the  pipe.  The  die  remains  in 
the  water  bath  until  the  "  singing  "  has  stopped,  about  50  to  90 
seconds,  and  is  then  cooled  in  oil  until  cold.  The  hardened  die  is 
later  ^tempered  in  oil  to  435°  F. 


376  STEEL  AND   ITS  HEAT  TREATMENT 

Many  alloy  steels  have  been  experimented  with  in  recent  years 
for  the  purpose  of  increasing  the  production  of  forgings  from  a 
given  impression,  thus  avoiding  the  loss  of  time  and  expense  incurred 
in  redressing  the  die-blocks.  A  chrome  nickel  steel  containing 
about  0.50  to  0.60  per  cent,  carbon,  0.50  per  cent,  chrome  and  1.50 
per  cent,  nickel  has  been  found  to  give  most  economical  results. 
These  die  blocks  are  hardened  and  tempered  in  the  usual  way, 
using  a  temperature  of  1400°  for  the  hardening  heat.  If  the  carbon 
content  runs  above  0.60  or  0.65  per  cent,  it  has  been  the  author's 
experience  that  cracking  during  or  directly  after  hardening  may 
result.  These  blocks  are  greatly  improved,  not  only  in  the  length 
of  service  to  be  obtained,  but  also  in  the  elimination  of  warpage 
during  hardening,  and  of  danger  of  cracking,  by  giving  the  block, 
before  machining,  a  full  heat  treatment  and  toughening  or  annealing ; 
blocks  which  approximate  the  composition  noted  should  be  quenched 
in  oil  from  a  temperature  of  1400°  to  1450°  F.,  and  then  full  annealed 
at  about  1250°  F.  Such  a  treatment  gives  excellent  results,  and  will 
also  show  up  any  defects  such  as  pipes,  seams,  etc.,  before  the  expen- 
sive machine  work  has  been  done. 

Dies  used  in  engraving  work,  and  in  the  jewelry  and  optical 
trades,  must  have  a  glass  finish,  both  in  smoothness  and  in  hardness. 
If  subjected  to  the  usual  quenching,  followed  by  sand-blast,  acid 
bath  or  cyanide,  a  large  amount  of  stoning  and  polishing  would  be 
required.  This  may  be  obviated  by  the  use  of  borax  or  boracic  acid 
in  the  following  manner.  Fill  the  matrix  with  powdered  boracic 
acid  and  place  near  a  fire  until  it  melts,  which  temperature  is  con- 
siderably below  the  tempering  point  or  color  of  the  steel.  Follow 
this  with  a  second  addition  of  boracic  acid  and  then  harden  as  usual. 
Although  the  salt  will  generally  come  off  in  the  quenching,  it  pro- 
tects the  polished  surface  of  the  die  and  does  not  interfere  with  the 
hardening.  In  case  the  salt  does  not  come  off  in  quenching,  it  may 
be  easily  removed  by  live  steam  or  boiling  water.  The  hardening 
may  be  done  by  complete  or  partial  submersion,  depending  upon  the 
thickness  and  general  design  of  the  die.  Engraving  dies  are  usually 
tempered  to  a  light  straw  color. 

Drills. — For  occasional  work  in  hardening  drills,  the  following 
procedure  may  be  used :  If  an  open  fire  is  the  only  available  source 
of  heat  for  hardening,  the  points  of  the  drill  should  be  kept  out  of 
the  hottest  part  of  the  fire  at  first,  drawing  them  in  as  the  upper 
parts  become  heated.  The  heat  should  extend  over  a  considerable 
portion  of  the  drill.  Quench  vertically  in  water,  and  keep  the  drill 


TOOL  STEEL  AND  TOOLS  377 

moving  up  and  down  so  that  there  is  no  abrupt  line  of  demarkation 
of  the  hardening.  If  the  drill  is  held  quietly  in  the  water,  fracture 
across  the  water  line  is  a  common  occurrence  when  the  drill  is  placed 
in  service.  Allow  the  drill  to  remain  in  the  water  until  the  im- 
mersed part  is  entirely  cold.  Remove,  brighten,  and  allow  the  heat 
in  the  shank  to  run  into  the  hardened  part  until  a  dark  straw  color 
appears  on  the  cutting  edge.  The  drill  should  then  be  immediately 
and  entirely  immersed  in  water.  If  there  is  not  sufficient  heat  in 
the  shank  to  bring  out  the  temper  color,  use  hot  ashes,  or  similar 
means.  The  drawing  operation  upon  hardened  drills  should  pref- 
erably be  carried  out  in  an  oil  or  salt  bath  subsequent  to  straight- 
ening; drawing  expensive  tools  to  color  is  poor  practice. 

It  is  always  advisable,  however,  if  an  open  fire  must  be  used  for 
heating,  as  noted  above,  to  heat  the  drill  in  a  pipe  or  tube  to  prevent 
the  direct  contact  of  the  fire  and  the  steel,  or  with  charcoal  to  prevent 
oxidation.  The  heating  should  be  done  slowly,  uniformly,  and  to  as 
low  a  temperature  as  is  possible  and  consistent  with  the  desired 
results. 

In  cases  where  a  large  number  of  drills  are  to  be  hardened,  it 
is  advisable  to  use  a  special  hardening  tank.  The  shape  of  the 
lands  of  the  drill  is  such  that  the  steam  formed  by  the  contact  of 
the  water  and  the  hot  metal  will  in  many  instances  prevent  the 
water  from  penetrating  to  the  flutes  and  properly  hardening  them, 
besides  having  a  similar  influence  on  the  end  of  the  drill,  which  will 
become  the  new  cutting  edge  as  the  point  is  ground  back.  This 
buffer  or  blanket  of  steam  may  be  eliminated  by  maintaining  a 
constant  flow  of  cold  water  into  the  grooves  and  against  the  end 
of  the  drills.  Perforated  pipes  may  be  placed  up  the  sides  of  the 
quenching  tank,  and  through  which  the  cold  water  is  forced  into  the 
grooves;  similarly,  a  jet  from  the  bottom  strikes  against  the  end  of 
the  drill. 

For  drills  for  holes  under  J  in.  in  diameter,  the  hardening  heat 
should  be  allowed  to  penetrate  only  through  the  cutting  part.  The 
drill  should  then  be  quenched  entirely  and  the  temper  drawn  to  suit 
the  work.  The  reason  for  not  allowing  the  hardening  heat  thor- 
oughly to  penetrate  to  the  core  of  the  drill  is  that  sudden  quenching 
of  a  small,  slender  piece  might  cause  severe  strains  to  be  set  up  in  the 
steel;  such  drills  also  require  a  tough  core  to  be  able  to  withstand 
the  torsional  effect  in  the  actual  drilling  operation.  Most  of  the  small 
drills  are  quenched  in  oil.  The  temper  color  is  usually  a  dark  straw. 
If  the  tempering  is  accomplished  by  placing  the  drills  upon  a  heated 


378  STEEL  AND   ITS  HEAT  TREATMENT 

bar,  the  cutting  parts  must  be  allowed  to  project  for  some  distance 
over  the  edge  of  the  hot  bar,  for  otherwise  the  heat  will  be  too  sud- 
denly applied. 

Milling  Cutters. — Under  this  class  are  included  cutters  of  varying 
description,  such  as  milling  cutters,  forming  cutters,  slotting  cut- 
ters, angle  cutters,  etc.  This  consists,  in  general,  of  a  cylindrical 
piece  of  steel  with  a  bore  through  the  center,  and  teeth  on  the  cir- 
cumference, sides,  or  both.  The  unequal  forces  of  contraction 
and  expansion  affect  these  tools  to  a  large  extent.  In  designing 
a  cutter,  as  large  a  mandrel  hole  as  is  possible  should  be  used,  as 
larger  holes  will  permit  the  steel  to  be  hardened  more  uniformly. 
If  the  mandrel  holes  are  standardized,  large  cutters  may  have  a 
part  of  the  sides  (in  the  absence  of  side  or  angular  teeth)  dished  or 
paneled  out  at  the  place  which  would  tend  to  garden  last,  that  is, 
half  way  between  the  two  circumferences. 

Great  care  should  be  used  in  heating  milling  cutters  for  harden- 
ing. The  heating  atmosphere  should  be  neutral  or  slightly  reducing 
to  protect  the  teeth.  If  an  open  fire  is  used,  the  fuel  should  not  be 
allowed  to  come  in  contact  with  the  cutter:  this  may  be  done 
by  resting  the  cutter  on  a  fire-brick  or  plate.  If  a  hearth  furnace 
is  used,  the  cutter  should  not  touch  the  floor  or  walls  of  the  furnace, 
but  should  be  supported  by  fire-bricks  or  other  suitable  methods. 
If  tongs  are  used  in  handling,  care  must  be  used  so  that  the  tongs 
do  not  touch  the  cutting  edges;  the  use  of  wires  is  better  practice. 
If  the  cutter  is  supported  on  bricks,  or  laid  on  plates,  it  must  be 
turned  repeatedly  in  heating  so  as  not  to  leave  any  unevenly  heated 
spots.  The  cutter  may  be  conveniently  held  in  the  quenching  bath 
by  using  a  small  round  bar  which  has  three  prongs  welded  to  one  end, 
and  which  extend  at  right  angles  to  the  axis  of  the  bar,  by  slipping 
the  other  end  of  the  bar  through  the  mandrel  hole  of  the  cutter ;  the 
latter  will  rest  on  the  prongs,  and  then  can  be  conveniently  lowered 
into  the  quenching  bath. 

Ordinary  cutters  are  best  hardened  by  the  use  of  two  small  cir- 
cular plates  of  a  diameter  slightly  greater  than  that  of  the  cutter, 
and  with  holes  bored  through  the  center  corresponding  to  the  size 
of  the  mandrel  hole  of  the  cutter.  One  plate  is  placed  on  each  end 
of  the  cutter,  and  the  whole  placed  on  the  suspension  tool  as  de- 
scribed above  and  immersed  vertically  in  the  quenching  bath. 
By  the  use  of  these  plates,  the  hardening  will  affect  the  steel  along 
the  entire  length  of  the  teeth  and  at  right  angles  to  the  center  line 
of  the  cutter.  This  will  also  eliminate  the  circular  fracture  or 


TOOL  STEEL  AND  TOOLS  379 

flaking  of  the  teeth  which  so  often  characterizes  milling  cutters 
subjected  to  uneven  cooling.  While  in  the  quenching  bath,  the  cut- 
ter should  be  moved  up  and  down  and  not  from  side  to  side ;  this  will 
permit  the  solution  to  pass  through  the  center  hole  and  give  an  evenly 
hardened  core.  The  combined  use  of  water  and  oil  ("  broken  hard- 
ening ")  in  the  following  manner  is  good  for  hardening  for  large 
cutters:  quench  in  water  until  the  "  singing  "  caused  by  the  water 
boiling  on  the  hot  steel  has  stopped,  and  then  immerse  in  oil  until 
cold;  warm  the  cutter  in  boiling  water  to  relieve  the  strains  and 
temper  when  convenient.  Pack-hardening  is  also  used  to  some  extent 
for  milling  cutters  in  order  to  prevent  oxidation;  in  this  case  each 
piece  should  be  quenched  separately.  Salt  baths  and  lead  baths  are 
also  used  for  heating.  One  of  the  main  points  to  be  observed  in 
quenching  milling  cutters  is  that  long  cutters  should  be  plunged 
vertically  and  thin  ones  edgewise. 

The  tempering  of  milling  cutters  is  often  done  by  the  insertion 
of  a  hot  rod  through  the  mandrel  hole  and  revolving  the  cutter  on 
it  until  the  proper  temper  color  is  obtained.  The  most  satisfactory 
results  are  to  be  obtained  with  the  use  of  an  oil  bath,  as  an  even  hard- 
ness can  be  best  obtained  in  this  manner.  Small  cutters  are  tem- 
pered to  a  light  straw  color,  or  yellowish-white.  For  medium-sized 
cutters  a  good  straw  color  may  be  used.  Very  large  cutters,  on 
account  of  the  lesser  effect  of  the  hardening,  may  not  require  temper- 
ing, but  it  is  always  advisable  to  heat  them  in  boiling  water  to  make 
them  uniform  and  remove  the  hardening  strains. 

For  hollow  mills  it  is  not  necessary  to  heat  for  hardening  very 
much  above  the  teeth,  as  it  is  not  required  that  the  back  should  be 
hard.  Harden  with  the  teeth  upwards,  working  the  piece  up  and 
down  in  the  quenching  bath  to  get  the  solution  circulating  through 
the  hole. 

T-slot  milling  cutters  should  be  hardened,  not  only  through  the 
cutting  portion,  but  also  through  the  entire  length  of  the  neck, 
especially  if  this  is  of  small  diameter.  In  tempering,  the  cutting 
portion  should  be  drawn  to  a  straw  color  and  the  neck  to  a  blue  color. 

Files. — Before  the  file  blanks  can  be  ground  and  the  teeth  cut 
it  is  necessary  to  anneal  the  steel.  This  is  often  accomplished  by 
packing  the  blanks  in  air-tight  oblong  boxes  and  annealing  at  about 
1300°  to  1400°  F. 

Lead  baths  continue  to  be  most  used  as  the  heating  medium. 
Salt  baths  have  been  tried  with  varying  degrees  of  success,  but  in 
the  main  have  proven  unsatisfactory.  This  is  due  in  a  large  measure 


380  STEEL  AND  ITS  HEAT  TREATMENT 

to  the  fact  that  oxide  of  iron  (scale)  may  settle  upon  the  teeth  of  the 
file,  causing  soft  spots  when  hardened.  The  method  of  dipping  the 
file  into  a  solution  of  ferrocyanide  and  allowing  the  coating  to  dry 
upon  the  surface  of  the  steel  before  heating  has  been  tried.  The 
objections  to  the  use  of  this  method  are  that  a  decomposition  of  the 
ferrocyanide  will  yield  additional  iron  oxide  and  poisonous  fumes. 
Other  salts  of  a  harmless  character  have  been  tried  with  little  success. 
The  general  procedure  is  to  cover  the  file  with  a  paste  which  pro- 
tects the  edges  of  the  teeth  in  the  hardening  process,  heating  in  lead 
to  the  proper  temperature  (about  1400°  F.)  and  quenching  in  water 
in  a  vertical  position.  One  file-maker  uses  a  paste  made  of  the 
following  base:  ground  charred  leather,  2  parts;  table  salt, 4  parts; 
and  flour,  3  parts.  The  file  is  given  a  coat  of  this  paste,  which  is 
allowed  to  dry  before  heating.  It  is  said  that  the  melting-point  of 
this  paste  will  give  the  proper  hardening  temperature.  After  being 
hardened,  and  while  the  file  is  still  warm,  it  is  put  through  the  final 
straightening  process. 

Half-round  files  require  particular  attention  on  account  of  their 
tendency  to  warp :  before  hardening,  the  file  is  bent  back  on  a  fixed 
template  of  such  form  as  experience  has  shown  will  bring  the  file  to  a 
true  line  upon  hardening;  the  file  is  placed  again  in  the  template 
before  it  is  quite  hard,  strained  to  the  proper  degree,  and  water  is 
thrown  on  the  upper  surface  of  the  file  to  make  it  quite  cold  before 
the  strain  is  relieved;  the  file  is  then  entirely  quenched  and  will 
usually  return  "  to  the  true  "  after  the  final  hardening. 

After  the  final  straightening  the  files  are  "  scrubbed  "  to  remove 
the  paste,  and  are  then  washed  in  lime  water  and  dried  by  holding 
them  in  steam.  The  tang  is  then  toughened  or  "  blued  "  by  dipping 
it  into  a  special  bath  maintained  at  the  proper  temperature. 

File  steel  will  vary  in  carbon  from  0.90  to  1.60  per  cent.,  accord- 
ing to  the  size,  shape  and  use  of  the  file;  manganese  under  0.40  per 
cent.;  low  phosphorus  and  sulphur;  and  in  the  case  of  exceptionally 
good  files,  a  small  percentage  of  chrome.  Nickel  is  generally  con- 
sidered as  detrimental  to  files. 

Reichhelm  1  shows  the  detrimental  effect  of  heat  variations  in 
hardening  in  the  microscopic  photographs  of  two  fractures  of  the 
same  file  magnified  160  times.  This  file  is  one  of  the  highest  grade 
produced  in  Europe,  and  Fig.  204  shows  the  fracture  of  this  file  as 
imported,  while  Fig.  205  shows  a  fracture  of  the  same  file,  a  section 
of  which  was  rehardened,  after  the  exact  degree  of  heat  required 
1 "  Machinery/'  Dec.,  1914. 


TOOL  STEEL  AND  TOOLS 


381 


FIG.  204. — Photomicrograph  of  High-grade  Foreign  File. 
(Reichhelm.) 


X160. 


FIG.  205.— Photomicrograph  of  Same  File  Rehardened. 
(Reichhelm.) 


X160. 


382  STEEL  AND   ITS  HEAT  TREATMENT 

for  this  particular  steel  had  been  experimentally  determined.  Fig. 
204  therefore  shows  the  result  of  the  best  hardening  practice  in 
Europe,  aided  by  the  pyrometer,  while  Fig.  205  shows  the  hardening 
of  this  identical  file  by  the  correct  heat  automatically  maintained. 
That  any  number  of  files,  or  tools  of  any  kind,  can  be  hardened  so 
as  to  show  uniformly  the  excellent  fracture  exhibited  in  Fig.  204  is 
due  to  automatic  heat  control,  as  has  been  demonstrated  conclusively 
in  daily  practice  for  over  three  years. 

Both  of  the  photographs  of  fractures  have  been  pronounced 
excellent  by  competent  judges,  but  the  decidedly  finer  grain  and 
more  even  diffusion  of  the  carbon  shown  in  Fig.  205  produced  a 
difference  in  the  durability  of  the  file  teeth  of  nearly  50  per  cent.,  as 
compared  with  the  section  of  the  file  as  originally  hardened  and  shown 
in  Fig.  204. 

Punches  and  Dies. — Similar  to  all  round  tools,  punches  show  a 
great  tendency  to  flake  off  at  the  corners,  sometimes  a  whole  ring 
breaking  off.  Assuming  proper  heating,  this  may  be  overcome  to 
a  large  extent  by  means  of  a  water  spray.  Dies  of  intricate  shape 
and  possessing  sharp  angles  should  be  most  carefully  handled.  It 
is  often  advisable  to  fill  these  angles  with  a  little  putty  or  fire-clay 
to  lessen  the  hardening  effect  and  prevent  the  formation  of  quench- 
ing strains  at  right  angles  to  the  diagonal.  A  piece  of  binding  wire 
may  also  serve  for  this  purpose.  Dies  should  generally  be  quenched 
flat,  depending  upon  the  shape  of  the  piece.  Small  punches  should 
not  be  quenched  in  real  cold  water  on  account  of  the  liability  to 
cracking  under  sudden  cooling — an  oil  bath  or  lukewarm  water 
is  far  preferable.  Dies  or  ,any  press  tools  having  holes  near  the 
edge  should  always  have  these  holes  filled  with  clay  in  order  to  pre- 
vent cracking  or  too  great  hardening;  graphite  or  asbestos  may  also 
be  used  for  plugging  the  holes  for  stripper  or  guide  screws.  Punches 
and  dies  are  generally  tempered  to  about  a  straw  color,  the  depth  of 
this  varying  according  to  the  thickness  and  hardness  of  the  material 
to  be  punched.  The  tempering  may  be  carried  out  by  setting  the 
hardened  pieces  in  front  of  a  hot  furnace,  laying  on  hot  plates,  in 
oil  baths  or  in  hot  sand. 

Reamers. — Reamers  may  be  heated  in  lead  to  protect  the  cutting 
edge  from  the  direct  action  of  the  heat  and  oxygen.  The  lead  may 
be  prevented  from  sticking  to  the  tool  if  the  latter  is  brushed,  in 
the  case  of  small  reamers,  with  a  little  soft  soap.  Larger  reamers 
may  be  protected  with  a  paste  made  of  black  lead  and  water  or 
lampblack  and  linseed  oil,  both  of  which  should  be  allowed  to  dry 


TOOL  STEEL  AND  TOOLS  383 

on  the  tool  before  heating  for  hardening.  If  the  reamer  has  been 
hardened  by  the  use  of  water  alone,  and  is  larger  than  f  in. 
in  diameter,  it  is  advisable  to  hold  it  over  the  fire  directly  after 
being  removed  from  the  hardening  bath,  or  to  set  it  in  hot  water 
for  a  few  moments,  in  order  to  remove — as  far  as  possible — the 
strains  which  have  been  caused  by  the  hardening  process.  This 
should  always  be  done  in  the  case  of  shell  reamers  and  other  special 
reamers  of  any  considerable  size,  whether  the  quenching  medium 
has  been  oil  or  water.  Broken  hardening  is  most  excellent  for  tools 
of  this  description.  Large  fluted  reamers  require  to  have  only  the 
ribs  heated  to  the  proper  temperature,  and  then  quenched;  temper- 
ing will  not  then  be  required.  Ordinary  fluted  reamers  are  tempered 
to  a  yellowish  white  or  very  light  straw  color.  Six-sided  or  eight- 
sided  reamers  may  be  tempered  to  a  light  straw  color.  Square 
reamers,  triangular  reamers  and  half-round  reamers  may  be  tem- 
pered to  a  dark  straw  color,  due  to  the  fact  that  they  take  hold  of 
the  work  more  deeply  and  might  break  if  not  tempered  a  trifle 
softer. 

Half-round  reamers  should  not  be  quenched  vertically,  but  with 
the  half-round  side  at  an  angle  of  20  to  45  degrees  to  the  surface  of 
the  bath.  If  half-round  reamers  should  be  quenched  vertically, 
it  will  be  necessary  to  move  them  in  a  horizontal  manner  in  the 
direction  of  the  half-round  side  at  the  same  time  as  immersed  ver- 
tically. 

The  shanks  of  reamers,  taps,  drills,  broaches  and  similar  tools 
may  be  toughened  by  local  lead  tempering. 

Rings. — Rings,  collars  arid  hollow  tools  comprise  a  class  which 
require  great  hardness  in  the  inner  circumference  or  bore.  Quench- 
ing is  usually  done  by  means  of  allowing  a  full  stream  of  water  to 
flow  through  the  bore  if  it  is  quite  small,  or  in  the  case  of  tools  with 
larger  bores  the  insertion  of  a  small  pipe  with  a  series  of  holes  in  its 
circumference  and  through  which  a  continuous  stream  of  water  may 
be  forced,  forming  a  spray.  In  the  first  case  it  is  advisable  to  set 
the  tool  upon  an  asbestos-covered  washer  in  which  has  been  cut  a 
hole  slightly  larger  than  the  size  of  the  bore  of  the  tool  and  then 
apply  the  flange  end  of  the  water-supply  line  or  pipe  to  the  other 
opening.  Rings  or  collars  requiring  resistance  to  frictional  wear 
require  no  tempering.  Eccentric  rings  cannot  be  quenched  as  usual, 
as  the  relative  thickness  and  thinness  of  the  opposite  sides  would 
tend  to  give  unequal  expansion  and  contraction  and  cause  the  hole 
to  become  oval-shaped.  This  may  be  overcome  by  binding  a  small 


384 


STEEL  AND   ITS  HEAT  TREATMENT 


piece  of  iron  or  steel  to  the  thin  side,  heating,  and  quenching  ver- 
tically. 

Rivet  Sets. — Rivet  sets  should  never  be  quenched  directly  by 
immersion,  as  this  will  tend  to  make  the  edges  of  the  cup  break  off, 
the  center  to  remain  soft,  and  leave  a  line  of  great  weakness  between 


FIG.  206.— Rough  Method  of  Hardening  a  Rivet  Set. 

the  hardened  and  unhardened  parts.  A  simple  and  proper  method 
is  to  hold  the  cup  under  or  over  a  stream  of  water  so  that  the  latter 
will  impinge  directly  upon  the  bottom  of  the  cup,  as  shown  in  Fig. 
206.  If  there  are  numbers  of  rivet  sets  to  be  hardened,  an  arrange- 
ment of  clips  or  holders  under  each  tap  or  spigot  may  easily  be  set 
up.  The  tempering  may  be  carried  out  as  in  the  case  of  chisels 
(permitting  the  heat  in  the  shank  to  temper  the  cup)  or  the  shank 


TOOL  STEEL  AND  TOOLS  385 

may  be  placed  in  a  lead  bath  and  the  color  allowed  to  run  up  into 
the  cup;  the  rivet  set  should  then  be  entirely  quenched  to  prevent 
further  softening. 

Brearley  makes  the  following  points,  which  are  of  great  interest. 
Rivet  sets  may  have  a  short  life  due  to  the  wear  on  the  head,  which 
is  as  often  a  failure  as  that  produced  by  actual  fracture.  This  is 
pronounced  in  the  case  of  annealed  stock.  He  advises  hardening 
the  head  in  oil  before  hardening  the  cup.  Upon  reheating  for  hard- 
ening the  cup,  and  tempering,  a  steel  of  great  toughness  is  obtained, 
which  neither  splits  nor  forms  a  mushroom  head. 

Saws. — Saws  may  be  hardened  by  either  of  two  methods — direct 
immersion,  or  press  or  roll  hardening.  Circular  saws  may  be  heated 
by  enclosing  in  a  sheet-iron  case  or  box  between  layers  of  charcoal. 
Sufficient  space  for  expansion  must  be  allowed  to  eliminate  chance 
for  buckling.  Saws  may  also  be  heated  on  the  hearth  (if  level)  of 
any  type  of  hardening  furnace;  it  is  advisable,  however,  to  rest 
the  saw  on  an  iron  or  steel  plate  so  that  the  heating  may  be  gradual 
and  uniform.  The  greater  part  of  the  secret  for  the  successful  hard- 
ening of  saws  without  buckling  is  a  slow  and  careful  heating.  The 
saws  when  heated  to  the  proper  temperature  may  be  taken  out 
separately  with  tongs  or  a  J-shaped  hook.  For  direct  quenching 
they  should  be  immersed  edgewise  and  in  a  perfectly  vertical  position. 
It  is  better  to  have  a  thin  layer  of  oil  on  the  surface  of  the  water 
bath,  as  the  oil  will  ignite  when  the  hot  saw  enters  it,  forming  a 
thin,  protective  coating  on  the  saw7  and  thus  lessening  the  risk  of 
fracture.  Oil  alone,  or  oil  with  tallow  dissolved  in  it  will  give  suffi- 
cient hardness  for  thin  saws.  The  saws  may  also  be  placed  between 
lumps  of  tallow.  The  latter  (tallow)  is  a  better  hardener  than 
oil,  and  therefore  gives  a  greater  and  deeper  hardening.  Thin  cir- 
cular saws,  and  all  ordinary  saws  such  as  hack  saws,  hand  saws,  etc., 
may  be  most  satisfactorily  hardened  by  means  of  a  press.  A  com- 
mon and  inexpensive  method  is  to  have  two  cast-iron  plates  hinged 
together,  with  the  inner  surfaces  well  oiled  with  a  heavy  oil.  The 
hot  saw  is  placed  between  the  plates,  which  are  then  clamped  to- 
gether and  held  until  the  saw  is  cold.  Thin  band  saws  are  often 
hardened  by  means  of  rolls.  Circular  saws  for  metal  cutting  should 
be  tempered  to  a  dark  purple  color,  or  to  a  light  blue  for  wood  cutting. 
Hack  saws  require  tempering  to  a  light  purple  color, 


CHAPTER  XVII 
MISCELLANEOUS   TREATMENTS 

THE  following  examples  and  discussions  of  certain  heat-treat- 
ment methods  have  been  selected  in  an  arbitrary  manner  as  repre- 
sentative of  distinct  classes  of  work.  Many  others  might  just  as 
well  have  been  taken,  but  the  author  feels  that  those  selected  will 
perhaps  illustrate  in  a  general  way  some  of  the  many  problems 
which  arise  in  the  course  of  ordinary  heat-treatment  work. 

GEARS 

Gear-steel  Classification. — Automobile  and  similar  machine 
gears  may  be  broadly  grouped  according  to  the  method  of  heat 
treatment,  which,  of  course,  is  dependent  upon  the  composition  of 
the  steel.  Thus  the  three  classes  are: 

(1)  The  case-hardened  gear,  using  a  steel  of  low-carbon  content 
— generally  less  than  0.25  per  cent — and  depending  upon  the  case- 
carburizing  process  to  give  an  outer  layer  of  high-carbon  steel  and 
upon  the  subsequent  hardening  processes  to  produce  the  necessary 
wearing  surface  of  sufficient  hardness. 

(2)  The  oil-hardened  and  tempered  gear,  using  a  steel  of  the  alloy 
type  of  about  0.45  to  0.55  per  cent,  carbon. 

(3)  The  hardened  gear  (without  subsequent  tempering),  using  a 
steel  of  an  intermediary  carbon  content — about  0.30  per  cent. 

Requirements  of  Gears. — All  high-duty  gears  require  that  the 
steel  shall  be  readily  forgeable  and  machineable,  and  that  after 
treatment  it  shall  have  the  greatest  possible  hardness  with  the  least 
possible  brittleness.  In  this  connection  it  may  be  said  that  surface 
hardness  is  often  more  desirable  than  tensile  strength,  while  the 
question  of  brittleness  is  very  important  on  account  of  shocks. 

Case-hardened  vs.  Oil-tempered  Gears. — The  merits  or 
demerits  of  each  type  depend  largely  upon  the  point  of  view  and  the 
personal  experience  of  the  user.  Expert  opinion  may  differ  widely, 

386 


MISCELLANEOUS  TREATMENTS  387 

as  is  shown  by  the  following  excerpts  from  addresses  by  two  well- 
known  metallurgists.  One  says:1 

"  Several  years  of  observation  and  contact  with  the  trade  leads 
me  to  prefer  the  case-hardened  gear.  The  result  of  direct  tests 
upon  thousands  of  gears  of  both  types  leads  me  to  the  following  con- 
clusions: (1)  The  static  strength  of  a  case-hardened  gear  is  equal 
to  that  of  an  oil-hardened  gear,  assuming  in  both  cases  that  steel 
of  the  same  class  and  approximate  analysis  has  been  used  and  that 
the  respective  heat  treatments  have  been  equally  well  and  properly 
conducted.  (2)  Direct  experiments  proved  that  the  case-hard- 
ened gear  resists  shock  better  than  the  oil  tempered.  (3)  As  regards 
resistance  to  wear  the  same  type  is  incomparably  better,  although 
perhaps  not  as  silent  in  action. 

"  One  of  the  leading  makers  of  gears  has  proved  this  to  his  own 
satisfaction  of  late  by  an  arrangement  of  shafts  and  gears  whereby 
energy  is  transmitted  through  two  case-hardened  gears,  in  mesh 
with  each  other,  to  two  oil-hardened  gears.  The  gears  are  of  the 
same  size.  The  conditions  of  the  test  were  severe.  Five  sets 
of  the  oil-hardened  gea'rs  have  already  been  worn  out,  while  the 
original  case-hardened  gears  are  still  in  service  and  show  the  tools 
marks. 

"  Upon  the  part  of  many  there  is  a  strong  objection  to  case  hard- 
ening. In  nine  cases  out  of  ten  this  is  doubtless  due  to  the  fact 
that  the  case-hardening  operation  has  not  been  reduced  to  a  science. 
The  depth  of  case,  the  relation  of  case  to  core,  the  time  and  tem- 
perature to  produce  certain  results  and  the  exact  control  of  these 
conditions,  together  with  an  accurate  knowledge  of  the  material  to 
be  treated,  are  factors  that  enter  into  successful  case-hardening 
practice.  Further  points  in  favor  of  this  method  are  easier  machin- 
ing of  the  blanks,  and  at  least  equal  static  and  dynamic  properties 
with  kss  chance  of  injury  in  hardening." 

Then  here  is  the  opposing  argument:2  "  For  machine  tools, 
hardened  high-carbon  alloy  steel  gears  appear  to  be  preferable  to 
case-hardened  gears  for  a  number  of  reasons : 

"  1.  Physically  they  are  stronger  and  tougher  and  should  there- 
fore be  better  able  to  resist  sudden  impacts  and  extraordinary 
loads.  They  do  not  show  by  file  and  scleroscope  test  the  same 

1  J.  A.  Matthews,  "  Alloy  Steels  for  Motor  Car  Construction,"  Journ.  Frank- 
lin Inst,  May,  1909. 

2  From  a  paper  by  J.  H.  Parker,  before  National  Machine  Tool  Builders' 
Assoc. 


388  STEEL  AND   ITS   HEAT  TREATMENT 

degree  of  hardness  as  case-hardened  gears,  but,  nevertheless,  with 
proper  design,  the  dense-grained  gear-tooth  resists  wear  more  satis- 
factorily, as  was  demonstrated  recently  by  the  examination  of  a 
motor-car  transmission  that  had  covered  over  100,000  miles.  The 
high-carbon  steel  gears  in  this  car  still  showed  the  original  tool  marks. 
Not  long  ago  a  designer  of  machine  tools  commented  on  the  ap- 
parent softness  of  some  hardened  high-carbon  gears,  but  found  after 
several  months  of  hard  service  that  they  still  showed  tool  marks, 
thus  proving  hardness  ample  for  wear. 

"2.  In  service,  especially  for  clash  gears,  the  superiority  of 
these  gears  is  most  marked.  On  the  clashing  faces,  case-hardened 
gears  are  likely  to  have  the  hard  case  chipped  off,  thereby  exposing 
the  soft  core  to  the  impact  of  clashing.  The  hard  chips  fall  into  the 
gearing  and  may  find  their  way  into  bearings,  thus  causing  trouble. 
High-carbon  steel  gears  with  a  uniform  hardness  throughout  do  not 
chip,  nor  do  they  '  dub  over.' 

"  3.  The  heat  treatment  of  high-carbon  steel  gears  is  much 
simpler  than  that  required  for  proper  case  hardening.  It  is  shorter, 
less  costly  and  produces  a  more  uniform  product,  and  as  the  gear 
is  heated  but  once  for  hardening,  as  compared  with  three  times  for 
case  hardening,  the  finished  gear  is  certain  to  be  freer  from  warpage. 
The  cost  of  proper  case  hardening  is  not  generally  appreciated,  but 
it  has  been  found  that  a  case-hardening  steel  must  cost  three  to 
four  cents  per  pound  less  than  a  regular  high-carbon  hardening 
steel,  if  finished  gears  made  from  both  materials  are  to  cost  the 
same. 

"  With  all  heat-treated  gears,  little  points  in  design  are  impor- 
tant. The  gear-teeth  should  not  be  undercut,  for  if  the  section  at 
the  root-line  is  smaller  than  at  the  pitch-line,  greater  hardness  and 
brittleness  is  produced  where  least  desired.  Great  differences  in 
section  should  be  avoided  wherever  possible,  so  as  to  do  away  with 
excessive  warpage.  Sharp  edges  and  angles,  even  in  key-ways,  are 
the  cause  of  internal  hardening  strains  which  frequently  result  in 
failures;  hence,  wherever  possible,  a  fillet  should  be  used  in  place 
of  a  sharp  angle." 

Case-hardened  Gears— Treatment. — The  steel  for  a  case-hard- 
ened gear  should  be  low  in  carbon,  preferably  under  0.25  per  cent.; 
should  be  carburized  so  as  to  produce  a  case  of  a  depth  of  about 
^T  or  ^  inch  and  contain  a  maximum  carbon  concentration  of 
about  0.9  per  cent.;  and  should  then  be  suitably  heat  treated. 
Since  the  principles  of  case  hardening  have  been  described  elsewhere, 


MISCELLANEOUS  TREATMENTS  389 

it  will  be  necessary  here  only  to  outline  the  process,  which  is  as 
follows : 

(Gear  blank). 

1.  Anneal. 

2.  Rough  machine  to  approximate  size 
(3.  Light  re-anneal.) 

4.  Finishing  machine. 

5.  Carburize  at  about  1600°-1650°  F. 

6.  Cool  slowly  in  carburizing  box. 

7.  Reheat  and  oil  quench  from  1550-1625°  F. 

8.  Reheat  and  oil  quench  from  1350-1425°  F. 
(9.  Temper,  if  desired,  to  not  over  400°  F.) 

The  temperatures  given  are  only  approximate,  depending  upon  the 
analysis  of  the  steel,  the  mass  of  the  steel,  the  results  desired,  and 
various  other  factors.  Nos.  3  and  9  may  be  omitted  if  desired. 

Oil-hardened  Gears — Treatment. — For  the  higher-carbon  steels 
used  for  oil-hardened  gears  it  is  always  advisable  to  give  the  gear 
blanks  a  preliminary  treatment  to  develop  the  highest  qualities  of 
the  alloy  steels  and  the  greatest  uniformity  in  their  physical 
properties.  This  treatment  will  also  give  the  greatest  "  softness  " 
of  which  the  steel  is  capable.  This  preliminary  treatment  (before 
machining)  is: 

1.  Quench  in  oil  from  about  150°  to  200°  F.  over  the  criti- 

cal range. 

2.  Quench  in  oil  from  about  50°  F.  over  the  critical  range. 

3.  Anneal  at  a  temperature  about  75°  F.   under   the  criti- 

cal range. 

If  this  preliminary  treatment  is  not  given,  the  gears  blanks  should 
be  given  a  thorough  annealing.  The  slight  reanneal  after  rough 
machining  and  before  the  final  cut  is  optional;  it  always  helps, 
however. 

The  final  treatment  consists  in  an  oil-hardening  and  tempering 
process.  For  the  majority  of  alloy  steels  this  quenching  is  done 
from  a  temperature  about  50°  F.  over  the  critical  range;  in  the  case 
of  chrome  vanadium  steels,  however,  the  best  results  are  generally 
obtained  by  the  use  of  a  higher  temperature.  The  temperatures 
generally  used  for  the  standard  types  of  alloy  steels  for  automobile 
gears,  approximating  0.45  to  0.55  per  cent,  carbon,  are  about  as 
follows : 


390  STEEL  AND   ITS  HEAT  TREATMENT 

Chrome  nickel  steels: 

1.5    per  cent,  nickel,  0.5    per  cent,  chrome,  1400°  F. 
1.75  per  cent,  nickel,  1.0    per  cent,  chrome,  1425 
3.0    per  cent,  nickel,  0.75  per  cent,  chrome,  1375 
3 . 5    per  cent,  nickel,  1 . 5    per  cent,  chrome,  1400 

Nickel  steels: 

3.5  per  cent,  nickel 1400°  F. 

5.0  per  cent,  nickel 1375 

Chrome  vanadium  steel: 

Type  "  D  "  (1 . 0  per  cent,  chrome,  0.8  per  cent. 

manganese,  0.16  per  cent,  vanadium) 1575°  F. 

Silico-manganese  steel : 

1.5  per  cent,  silicon,  0.7  per  cent,  manganese  1550°  F. 

The  usual  precautions  should  be  observed  such  as  uniform  and 
thorough  heating,  protection  from  oxidation,  etc.  Further,  the 
gear  should  be  quenched  in  the  direction  of  its  axis  so  that  the  oil 
can  be  made  to  circulate  around  the  teeth,  etc.  The  notes  given 
under  "  Milling  Cutters  "  1  might  also  be  of  interest  in  their  bearing 
upon  gear  treatment. 

The  tempering  is  usually  done  at  a  temperature  of  400°  F.  or 
upwards,  depending  upon  the  nature  of  the  steel  and  upon  the 
results  desired.  It  should  again  be  stated  that  a  longer  tempering 
at  the  lower  temperature  is  preferable  to  a  quicker  and  shorter  tem- 
pering at  a  higher  temperature.  Thus,  if  a  gear  were  to  have  the 
temper  drawn  quickly,  the  teeth,  which  should  be  the  hardest,  will 
be  softer  than  the  hub,  which  will  remain  brittle;  with  a  longer 
heating  at  a  lower  temperature  this  will  not  be  the  case,  since  the 
whole  gear  will  have  responded  throughout.  Similarly,  for  these 
reasons,  it  is  inadvisable  to  temper  gears  "  by  color,"  but  to  use  an 
oil  bath  or  a  mixture  of  low  melting-point  salts. 

For  gears  made  of  alloy  steel  with  only  about  0.30  per  cent, 
carbon  the  tempering  operation  is  usually  omitted.  It  is  always 
best,  however,  to  reheat  the  oil-quenched  gears  in  boiling  water  for 
a  short  time  in  order  to  remove  the  hardening  strains;  such  treat- 
ment will  have  little  or  no  influence  on  the  hardness  and  strength. 
The  quenching  temperature  for  such  steels  will  of  course  be  higher 
by  some  50°  or  75°  than  that  given  under  the  0.45-0.55  per  cent, 
carbon  steels. 

iCf.Ch.XVI. 


MISCELLANEOUS  TREATMENTS  391 

SPRINGS 

The  usual  analysis  for  carbon  steel  springs  is  approximately: 

Carbon 0.90  to  1 . 10  per  cent. 

Manganese under  0.40 

Phosphorus under  0. 04 

Sulphur under  0. 04 

Silicon up  to  0.25 

It  is  dangerous  to  allow  the  percentage  of  carbon  to  run  up  to 
1.25  per  cent,  (as  is  sometimes  done),  on  account  of  the  possibility 
of  the  formation  of  free  cementite,  which  is  an  extremely  brittle 
constituent.  A  crack  might  easily  start  in  an  area  of  cementite  and 
when  once  started  would  follow  through  the  cementite  to  the  outer 
surface.  Lower  carbons  would  preclude  the  presence  of  free  cement- 
ite. Finely  divided  cementite  would  also  be  less  dangerous, 
and  this  could  be  obtained  by  hardening  at  a  lower  temperature 
(about  1400°  F.),  since  crystallized  and  granular  cementite  can 
only  be  obtained  by  heating  for  a  prolonged  time  at  a  high  temper- 
ature. 

Aside  from  improper  analysis,  the  majority  of  spring  failures  and 
troubles  may  be  laid  to  abnormally  high  temperatures  for  heating 
for  fitting  followed  directly  by  quenching  from  whatever  temper- 
ature the  steel  may  happen  to  be  at;  and  then,  as  if  this  were  not 
bad  enough,  to  temper  by  "  flashing."  From  general  knowledge  it 
appears  that  the  maker  of  springs  has  not  kept  pace  with  improve- 
ments in  spring  steel  and  with  the  increased  severity  of  the  duty 
expected  of  springs. 

The  old  practice  of  high  temperatures  and  of  forming  and 
hardening  springs  with  a  single  heating  cannot  be  persisted  in  if 
maximum  quality  and  service  are  to  be  secured.  The  "  practical  " 
spring-fitter  generally  heats  the  steel  to  about  as  high  a  temperature 
as  it  will  take  without  burning.  Its  effect  upon  the  structure  of 
of  steel  has  been  explained  in  preceding  chapters,  and  also  above  in 
its  relation  to  very  high-carbon  spring  steel. 

But  even  assuming  that  the  proper  temperatures  have  been  used 
in  fitting,  the  time  taken  to  go  through  the  forming  operation  is 
sufficient  to  give  the  steel  a  chance  to  cool  down  to  a  temperature 
which  will  not  give  the  most  satisfactory  results  in  hardening. 
The  steel  is  not  of  uniform  temperature  over  its  length  so  that,  if  it 
be  quenched  directly  after  forming,  it  will  probably  lock  up  internal 


392  STEEL  AND   ITS  HEAT   TREATMENT 

strains  of  uncertain  magnitude — to  say  nothing  of  the  insufficient 
hardening  if  the  temperature  be  under  that  of  the  critical  range. 
In  other  words,  the  spring  should  be  put  back  in  the  furnace  again  (it 
being  generally  preferable  that  the  maximum  temperature  for  forming 
shall  be  the  same  as  that  required  for  hardening)  and  reheated  for 
a  few  minutes  so  that  it  will  be  heated  uniformly  throughout  at  the 
right  temperature.  If  high  temperatures  have  been  used  for  form- 
ing it  will  be  advisable  to  allow  the  steel  to  cool  to  a  temperature 
under  that  of  the  Ar  range  before  reheating  for  hardening;  if  this 
is  not  done  the  steel  will  retain  the  coarse  grain-structure  character- 
istic of  the  high  heat  for  forming.  If  it  is  found  that  the  steel 
departs  from  its  shape  at  all  during  this  reheating,  it  may  be  put 
through  the  rolls  again  previous  to  quenching,  the  time  occupied 
being  small  compared  with  that  for  the  original  bending.  The 
spring  should  then  be  quenched  in  some  good,  heavy  tempering  oil. 

For  drawing  the  temper  it  is  never  advisable  to  use  the  process 
known  as  "  flashing."  The  practice  of  replacing  the  steel,  after 
quenching,  in  a  high  temperature  furnace  until  the  outside  of  the 
steel  reaches  the  desired  temperature  is  one  which  cannot  be  too 
strongly  denounced,  because  of  the  impossibility  of  uniform  treat- 
ment. No  time  is  allowed  for  the  heat  to  soak  to  the  center,  with 
the  result  that  the  hardness  increases  from  the  outside — a  most 
undesirable  condition.  All  spring  steel  should  be  drawn  back  in  a 
suitable  low-temperature  furnace  maintained  at  the  proper  temper- 
ature. The  steel  should  be  kept  in  the  furnace  for  a  time  sufficient 
to  allow  of  a  uniform  heating  throughout.  Lead  baths  and  salt 
baths  are  also  used  considerably  for  this  work. 

The  proper  temperatures  for  treating  carbon  spring  steel  have 
been  given  considerable  attention  by  the  American  Society  for 
Testing  Materials.  Their  experiments  were  made  with  test  speci- 
mens If  by  f  by  14  ins.  long  with  straight  edges,  and  analyzing 
about  1.10  per  cent,  carbon.  The  results  of  these  tests  (1911) 
are  given  in  the  tables  on  page  394. 

It  is  apparent  that  at  a  quenching  temperature  of  1500°  F.  the 
maximum  results  are  obtained  with  a  drawing  temperature  of  about 
600°  F.,  while  with  a  quenching  temperature  of  1650°  F.  the  maxi- 
mum elastic  limit  was  found  with  a  drawing  temperature  of  about 
800°  F.  In  the  former  group,  Series  A,  1500°-600°  F.,  it  was 
found  that  the  angle  of  bend  at  rupture  showed  an  average  of 
slightly  over  59°,  there  being  considerable  variation  between  the 
specimens;  while  in  the  second  group,  Series  B,  1650-800°  F.,  the 


MISCELLANEOUS  TREATMENTS 


393 


average  angle  was  slightly  over  103°,  without  any  specimen  going 
below  76°.  These  results  are  particularly  interesting  in  view  of  the 
fact  that  the  critical  range  of  these  steels  is  about  1350°  F.,  and 
that  one  would  naturally  expect  that  a  temperature  of  about  1400° 
F.,  i.e.,  slightly  over  the  critical  range,  would  give  the  best 
results.  Whether  or  not  such  would  show  up  in  vibratory  tests  is 
a  question  which  should  be  given  attention. 

TRANSVERSE,  HARDNESS  AND  BENDING  TESTS  OF  CARBON  SPRING  STEEL 
Series  A,  Quenched  in  oil  from  1500°  F. 


Hardness. 

Bend  Test, 

Temper 
Drawn  to 

Elastic  Limit, 

(transverse) 

Scleroscope. 

Angle  Bent 
through  at 

Deg.  F. 

Lbs.  per  Sq.  In. 

T)f*{«"t  All 

Rupture, 

On  Flat. 

On  Edge. 

-tsrineii. 

Deg. 

425 

129,137 

48.5 

47 

370 

181 

600 

136,440 

46 

50.5 

388 

60 

835 

131,017 

43.5 

49.5 

351 

86 

1025 

96,852 

34.5 

39 

268 

152 

1230 

105,400 

34 

37 

282 

167 

Series  B,  Quenched  in  oil  from  1650°  F. 


450 

130,922 

46 

52.5 

394 

90 

625 

134,232 

43 

57 

371 

82 

820 

141,147 

46 

56 

389 

104 

1025 

126,320 

42 

50 

371 

108  . 

1210 

83,457 

31 

36.5 

260 

180 

ALLOY    STEEL   SPRINGS 

The  service  conditions  to  which  automobile  springs  are  sub- 
jected are  extremely  severe,  for  they  have  to  sustain  the  shocks  at 
speed  of  the  irregularities  of  the  ordinary  highway,  built  for  slow- 
moving,  horse-drawn  vehicles.  The  necessity  for  high  elastic  limit, 
combined  with  great  toughness  and  anti-fatigue  qualities,  make  the 
use  of  alloy  steel  almost  mandatory. 

The  alloy  steels  in  use  are  of  the  same  analysis  of  those  previously 
given  under  the  heading  of  "  Oil-hardened  and  tempered  Gears  " 
(q.v.).  The  quenching  temperatures  are  likewise  the  same  as 
there  given,  but  the  drawing  temperatures  are  higher — generally 
from  850°  to  1025°  F.  As  far  as  static  strength  is  concerned,  the 
majority  of  the  now  common  alloy  compositions  will  give  about 
the  same  test  values,  approximately : 


394  STEEL  AND  ITS  HEAT  TREATMENT 

Tensile  strength,  Ibs.  per  sq.  in. ...  190,000  to  250,000 

Elastic  limit,  Ibs.  per  sq.  in 170,000  to  225,000 

Elongation,  per  cent,  in  2  ins 15  to  6 

Reduction  of  area,  per  cent .  45  to  20 

Some  of  the  alloy  steels,  and  particularly  the  chrome  vanadium 
type,  require  annealing  before  shearing.  The  chrome  vanadium 
steels  used  for  springs  are  readily  susceptible  to  "  temper,"  and  it 
is  likely  that  the  rapid  air  cooling  of  small  flats  after  they  leave  the 
rolls  will  cause  them  to  be  brittle,  thus  giving  a  great  amount  of 
trouble  in  shearing.  The  annealing  of  this  chrome  vanadium  steel 
is  done  by  bringing  the  steel  up  to  a  full  cherry-red  heat  in  the 
furnace  (about  1475°  F.)  and  allowing  it  to  cool  slowly  after  being 
maintained  at  this  temperature  for  a  sufficient  time  to  allow  of  uni- 
form heating. 

The  new  steels  cannot  be  handled  just  like  the  old  carbon  steel 
springs  and  still  obtain  from  them  the  maximum  development  of 
their  powers.  However,  the  new  steels,  being  in  general  lower  in 
carbon,  will  stand  much  abuse  in  heat  treatment  and  still  pro- 
duce springs  of  quality  undreamed  of  a  decade  ago.  While  as  a 
class  spring-makers  have  been  driven  to  the  use  of  alloy  steels,  they 
have  not  as  a  class  been  forced  to  handle  them  scientifically. 

Alloy  steels  especially  should  not  be  heated  any  higher  for  form- 
ing than  is  absolutely  necessary.  Then  they  should  always  be 
reheated  to  the  proper  temperature  for  quenching  in  order  to  make 
sure  that  the  entire  steel  is  uniformly  heated  throughout  to  that 
temperature,  which  must  be  exact.  The  same  remarks  about  tem- 
pering as  given  under  carbon  steel  springs  likewise  apply  here,  and 
with  added  emphasis. 

OIL-WELL   BITS 

Bits  used  for  drilling  oil  wells,  gas  wells,  etc.,  represent  that 
class  of  large  implements  requiring  "  end  heats."  The  hardening  of 
these  bits  is  necessarily  an  operation  to  be  carried  out  in  the  field, 
since  the  bits  require  a  more  or  less  frequent  dressing  and  must  be 
rehardened  after  each  heating.  An  extremely  hard  end  and  face, 
together  with  a  strong,  tough  core  and  shank  are  the  principal 
requirements  for  this  work. 

About  6  or  8  ins.  of  the  bit  is  carefully  heated  in  the  fire  (usually 
a  common  blacksmith  forge),  to  the  proper  temperature — usually 
about  1500°  F.  Higher  temperatures  should  not  be  used  unless 
absolutely  required  by  the  nature  of  the  steel.  Any  scale  should  be 


MICELLANEOUS  TREATMENTS 


395 


carefully  and  quickly  brushed  off  before  quenching.  The  bit  is 
then  removed  from  the  fire  and  allowed  to  rest  in  a  bucket  of  coarse 
salt  for  a  second  or  two.  This  salt  treatment  may  be  omitted,  but 
it  undoubtedly  gives  better  results;  the  direct  use  of  brine  is  gen- 
erally too  severe  for  most  bit  steels. 

A  box  or  trough  should  previously  be  fitted  with  a  wooden  grating 
made  of  slats,  the  top  of  which  will  be  about  3  or  4  ins.  under  the 
surface  of  the  water  in  the  box.  Some  drillers  add  vitriol  to  the 
water  quenching  bath  to  obtain  a  greater  hardness.  The  bit  should 
then  be  quickly  lowered  vertically  into  the  cold  water  until  it  rests 
upon  the  wooden  grating,  and  should  be  allowed  to  remain  there 
until  cold. 

The  precautions  to  be  observed  are:  (1)  Lower  vertically,  in 
order  to  obtain  an  equal  hardness  on  both  faces  of  the  bit;  (2)  do 
not  quench  to  a  greater  depth  than  3  or  4  ins. ;  (3)  do  not  move  the 
bit  nor  splash  the  heated  part  of  the  shank  with  water;  (4)  allow 
the  steel  to  remain  in  the  water  until  cold,  generally  over  night. 
Although  the  surface  of  the  water  bath  may  steam,  it  will  generally 
be  found  that  directly  beneath  the  surface  the  water  is  cold, 
and  likewise  the  end  of  the  bit.  Splashing  the  heated  part  of  the 
bit  with  water  has  a  tendency  to  draw  the  temper  of  the  faces. 
Immersion  to  a  greater  depth  than  3  or  4  ins.  is  apt  to  give  a  soft 
bit.  If  these  precautions  are  carefully  observed,  and  the  steel  is 
of  the  right  analysis,  a  bit  with  a  glass-hard  surface  and  a  strong, 
tough  core  will  be  obtained.  Such  bits  require  no  tempering,  and 
should  not  chip  off. 

Oil-well  bit  steel  will  vary  between  0.50  and  0.80  per  cent,  car- 
bon and  manganese,  low  phosphorus  and  sulphur,  up  to  0.25  per  cent, 
silicon,  and  the  addition  of  about  0.5  per  cent,  chrome  for  the  lower 
carbons.  The  chrome  bit  steel,  if  of  the  proper  carbon-manganese- 
chrome  composition,  will  undoubtedly  give  the  best  service.  The 
following  analyses  are  characteristic  of  American  oil-well  bits  used 
and  giving  good  service : 


Carbon. 

Manganese. 

Phosphorus. 

Sulphur. 

Silicon. 

Chrome. 

0.73 

0.61 

0.017 

0.030 

0.14 

0.59 

0.17 

0.010 

0.015 

0.13 

0.83 

0.65 

0.010 

0.021 

0.13 

0.60 

0.51 

0.012 

0.019 

0.01 

0.56 

0.54 

0.53 

0.007 

0.021 

0.006 

0.51 

0.49 

0.56 

0.010 

0.016 

0.008 

0.52 

396  STEEL  AND   ITS  HEAT  TREATMENT 


SAFE   AND    VAULT   STEEL 

Safe  and  vault  steel  may  be  taken  as  representative  of  that  class 
of  material  involving  different  steels  welded  together,  but  for  which 
the  proper  treatment  of  one  analysis  will  be  sufficient  for  both. 
Steel  for  safes  and  vaults  consists  of  alternate  layers  of  soft  and  hard 
steel,  and  is  known  to  the  trade  as  "  three-ply,"  "  five-ply,"  etc. 
By  having  these  alternate  layers  there  is  obtained,  under  suitable 
treatment,  a  metal  which  will  have  sufficient  ductility  (due  to  the 
soft  layers)  to  resist  explosive  forces,  and  at  the  same  time  be  im- 
penetrable to  drilling,  sawing  or  other  machine  operations  (due  to 
the  "  hard  center").  The  soft  layers  are  made  of  ordinary  low- 
carbon  or  "  soft ."  steel,  while  the  hard  centers  will  analyze  about 
0.85  to  1.05  per  cent,  carbon  and  manganese,  with  or  without  the 
addition  of  chrome. 

The  plate  is  first  machined  or  ground  to  size  and  the  necessary 
holes  drilled,  threaded,  and  plugged  with  fire-clay  for  protection. 
The  plate  is  then  placed  in  a  suitable  heat-treatment  furnace,  and 
thoroughly  heated  to  1400°  to  1500°  F.,  depending  upon  the  compo- 
sition of  the  hard  layer.  It  is  extremely  important  that  ample 
time  be  allowed  for  the  heat  to  penetrate  and  thoroughly  heat 
the  high-carbon  steel,  for  it  is  upon  the  hardness  of  these  layers 
that  the  full  value  of  the  finished  plate  will  depend.  The  major- 
ity of  the  cases  in  which  the  necessary  hardness  was  not  obtained 
which  the  author  has  investigated  have  been  due  to  an  insufficient 
length  of  heating  rather  than  to  any  fault  in  the  analysis  of  the 
steel. 

The  plate  is  then  quickly  removed  from  the  furnace  by  a  crane 
or  hoist  and  quenched  in  cold  water.  As  the  hardness  is  largely 
dependent  upon  the  rapidity  with  which  the  steel  is  cooled  through 
the  critical  range,  arrangements  should  be  made  to  obtain  a  constant 
supply  of  cold  water  in  contact  with  the  steel  during  the  quenching 
operation.  If  the  quenching  is  done  in  a  tank,  the  inlet  supply  should 
be  large  enough  always  to  keep  the  water  cold — the  warm  water 
being  taken  away  from  near  the  top  of  the  tank.  In  this  case  the 
plate  is  quenched  vertically;  particular  care  should  be  used  in  getting 
the  whole  plate  into  the  water  as  quickly  as  possible,  and  in  an  ab- 
solutely vertical  position,  if  warpage  is  to  be  avoided.  As  soon  as 
the  initial  immersion  is  accomplished  the  plate  may  be  swung  to 
and  fro  in  the  tank  to  aid  in  the  heat  removal.  Other  plants  quench 
by  means  of  water  sprays,  the  plate  being  supported  on  a  horizontal 


MISCELLANEOUS  TREATMENTS  397 

rack;  with  this  method  of  cooling  the  water  supply  should  be  suffi- 
cient to  remove  the  steam  as  soon  as  it  is  formed. 

The  plates  are  not  tempered  or  drawn.  Specifications  require 
that  the  best  high-speed  steel  drill  shall  not  penetrate  the  hard- 
center  layers. 

STEEL  CASTINGS 

In  the  mad  rush  for  alloy  steels  and  their  heat  treatment  but 
little  attention  has  been  given  to  the  treatment  of  steel  castings. 
And  yet  there  is  an  opportunity  for  as  great,  if  not  greater,  improve- 
ment in  these  parts  as  in  forged  or  rolled  sections.  All  steel  castings 
should  be  annealed  or  oil  treated,  not  only  to  remove  the  casting 
strains,  but  also  to  get  the  metal  into  the  best  possible  condition. 
Due  to  the  method  of  fabrication,  the  rapid  cooling  of  thin  sections 
and  the  slower  cooling  of  adjacent  thicker  sections  must  inevitably 
produce  casting  strains  of  a  more  or  less  intense  nature.  Similarly 
and  coincidently ,  the  structure  of  the  metal  must  inherently  be  poor : 
the  grain  will  be  coarse  instead  of  fine  and  "  silky,"  the  metal  will 
tend  to  have  low  ductility  and  brittleness,  and  the  physical  proper- 
ties of  the  steel  as  a  whole  will  vary  considerably.  Unlike  forgings 
and  rolled  sections,  castings  are  not  generally  subjected  to  any 
reheating  and  elaboration,  so  that  the  metal  must  have  those  prop- 
erties characteristic  of  moderate  cooling  from  high  temperatures. 

Thus  the  usual  specifications  for  steel  castings,  in  which  the  low 
ductility  will  be  apparent,  will  call  for: 

Tensile  strength,  Ibs.  per  sq.  in 85,000 

Elastic  limit,  Ibs.  per  sq.  in 45,000 

Elongation,  per  cent,  in  2  ins 12 

Reduction  of  area,  per  cent 18 

Even  the  now  common  addition  of  titanium  or  vanadium  will  not 
serve  to  eliminate  entirely  the  necessity  for  subsequent  treatment. 
Annealing,  or  better  still,  a  full  heat  treatment,  is  mandatory. 

Contrary  to  the  ideas  held  by  many  "  practical  "  hardeners,  the 
principles  of  treating  steel  castings  in  no  wise  differ  from  those  of 
steel  forgings  of  the  same  section  and  analysis.  The  main  difficulty 
encounterad  is  that  caused  by  the  length  of  time  required  for  the 
diffusion  of  the  ferrite  and  the  equalization  of  the  metal  as  a  whole. 
Castings  usually  require  considerable  time  for  this  to  take  place 
because  of  the  tendency  of  the  metal  to  return  to  its  original 
molecular  arrangement  and  structure  during  slow  cooling.  Thus 


398  STEEL  AND   ITS  HEAT  TREATMENT 

much  of  the  unsatisfactory  annealing  is,  technically  speaking,  due 
to  the  segregation  of  the  ferrite. 

It  is  therefore  necessary,  in  annealing  steel  castings,  to  (1)  heat 
well  over  the  upper  critical  range,  (2)  for  a  length  of  time  sufficient 
to  obliterate  entirely  the  previous  structure  and  crystallization, 
and  followed  by  (3)  slow  cooling.  The  proper  annealing  temperature 
for  the  ordinary  machinery  castings  will  be  between  1500°  and  1600° 
F.,  depending  upon  the  carbon  content. 

If  the  annealing  is  preceded  by  normalizing,  i.e.,  air  cooling  from 
a  temperature  considerably  above  the  upper  critical  range — say 
1800°  F. — the  length  of  time  required  for  the  subsequent  anneal 
will  be  considerably  shortened,  besides  improving  the  steel. 

For  castings  with  the  carbon  on  the  lower  side  of  0.25  or  0.30 
per  cent.,  or  for  castings  of  considerable  size,  air  cooling  from  about 
1600°  F.  will  usually  produce  good  results. 

The  best  method,  however,  is  that  of  oil  quenching  and  annealing 
or  toughening — either  with  or  without  a  previous  normalizing. 
The  castings  should  be  heated  as  directed  under  annealing,  quenched 
in  the  proper  manner  in  oil,  and  then  reheated  to  the  temperature 
which  will  give  the  combination  of  strength  and  ductility  desired.  A 
drawing  temperature  of  1250°  F.  will  produce  the  most  ductile 
steel. 

STEEL   WIRE1 

The  principal  heat  treatments  used  in  the  manufacture  of  wire 
are:  1,  annealing;  2,  patenting;  3,  hardening  and  tempering. 

Annealing  serves  to  accomplish  three  important  functions: 
1.  To  remove  the  effects  of  hardening  due  to  cold  work  in  wire 
drawing  or  cold  rolling,  thus  making  the  steel  ductile  and  soft. 
Annealing  for  this  purpose  covers  principally  the  low-carbon  wires, 
those  with  carbon  0.25  per  cent,  and  under.  2.  To  refine  grain- 
applied  principally  to  the  higher-carbon  rods  and  wires,  those  with 
carbon  0.30  per  cent,  and  over.  3.  To  obtain  definite  structure  in 
the  finished  material — applied  principally  to  the  higher-carbon  wires, 
those  with  carbon  0.30  per  cent,  and  over. 

When  a  steel  wire  rod  of  the  structure  shown  in  Fig.  207  is  sub- 
jected to  the  wire-drawing  process,  a  marked  change  in  the  grain 
structure  takes  place.  With  each  successive  draft,  the  grains  stretch 
out  in  the  direction  of  drafting  until  a  point  is  reached  when  the 

1  From  a  paper  by  J.  F.  Tinsley,  American  Iron  and  Steel  Inst.,  1914,  and 
The  Iron  Age,  May  28,  1914. 


MISCELLANEOUS  TREATMENTS  399 

grains  have  been  elongated  to  the  limit  of  their  ductility.  If  sub- 
jected to  further  strain  by  further  drafting  they  will  part  and  the 
wire  will  break.  Before  this  brittle  condition  is  reached,  therefore, 
it  is  necessary  to  heat  treat  the  wire  by  subjecting  it  to  what  is 
known  in  the  wire  business  as  a  "  process  annealing." 

The  effect  of  wire  drawing  in  elongating  the  structural  grain 
of  the  steel  may  be  seen  by  comparing  Figs.  207,  208  and  209.  Fig. 
207  shows  the  structure  of  the  rod  before  drawing;  Fig.  208  shows  the 
structure  after  a  15  per  cent,  reduction  from  the  rod;  and  Fig.  209, 
the  structure  after  a  60  per  cent,  reduction  from  the  rod.  All  of 
these  micrographs  represent  sections  taken  from  a  plane  parallel 
to  the  axis  of  tho  rod  or  wire,  not  cross-sections.  The  reason  for  the 
marked  difference  in  grain  shown  in  Figs.  207  and  209  may  be  grasped 
more  clearly  when  it  is  appreciated  that  Fig.  209  represents  a  wire 
reduced  in  the  wire-drawing  process  to  such  a  degree  that  it  has 
become  elongated  2J  times  the  original  length  of  the  rod. 

Process  or  "  works  "  annealing  consists  in  heating  the  wire  to  a 
certain  temperature,  maintaining  that  temperature  until  the  entire 
mass  of  steel  is  thoroughly  heated  through,  and  finally  cooling  down. 
In  the  most  common  of  all  annealing — that  to  remove  the  effects 
of  cold  work  such  as  drawing — it  is  not  necessary  to  reach  the 
critical  temperature,  which  is  1300°  F.,  or  higher,  depending  on  the 
carbon  content.  A  temperature  of  1100°  F.  is  entirely  sufficient  to 
relieve  the  strained  condition  of  the  grain  shown  in  Fig.  209.  Fig. 
210  shows  the  same  wire  that  is  depicted  in  Fig.  209  after  annealing 
at  a  temperature  below  the  critical  range. 

In  the  annealing  process  the  strained  and  elongated  grains 
shown  in  Fig.  209  break  up  and  rearrange  themselves  to  form  a 
new  grain  structure  as  shown  in  the  micrograph.  The  annealed 
steel  of  the  structure  shown  is  now  in  excellent  condition  to  with- 
stand further  cold  work  in  reducing  it  to  finer  sizes;  or,  if  already 
at  finished  size,  is  in  good  condition  to  meet  the  demands  of  annealed 
wire  service. 

The  effect  of  reduction  of  section  incident  to  wire  drawing  on 
the  tensile  strength  and  ductility  of  steel  wire,  and  the  marked 
change  brought  about  in  these  characteristics  by  annealing,  as  just 
outlined,  is  shown  in  Fig.  216.  This  table  is  based  on  drafting  and 
annealing  practice  in  reducing  a  low-carbon  steel  rod — in  this  case 
0.10  per  cent,  carbon — to  a  fine  size  of  wire.  It  will  be  noted  that 
between  80  per  cent,  and  90  per  cent,  reduction  from  the  rod  or 
annealed  wire  can  be  taken  before  annealing  is  necessary. 


400 


STEEL  AND   ITS  HEAT  TREATMENT 


It  is  found  in  practice  that  in  cold  drawing  from  a  soft  rod  or 
annealed  wire,  the  increase  in  tensile  strength  is  a  direct  function 
of  the  amount  of  cold  work,  almost  independent  of  other  conditions. 


FIG.  207.— Annealed  (0.08  Carbon)  Steel.     (Tinsley.) 

Annealing  practically  brings  the  rod  or  wire,  regardless  of  size, 
back  to  its  original  condition  with  regard  to  tensile  strength  and 
ductility.  It  will  be  noted  that  the  final  annealing  does  not  bring 


FIG.  208.— Steel  Wire  (O.OS  Carbon) 
Given  One  Draft;  15  per  cent. 
Reduction  from  Rod.  (Tinsley.) 


FIG.  209.— Steel  Wire  (0.08  Carbon) 
Given  Several  Drafts;  60  per 
cent.  Reduction  from  Rod. 
(Tinsley.) 


the  tensile  strength  as  low  as  previous  annealing.  This  is  due 
simply  to  the  fact  that  in  annealing  the  fine  sizes  it  is  usual,  in  order 
to  avoid  the  mechanical  sticking  of  the  wire  in  coils,  to  anneal  at 
slightly  lower  temperatures  than  in  ordinary  process  annealing. 


MISCELLANEOUS  TREATMENTS  401 

The  second  important  function  of  annealing  is  that  of  refining 
grain,  and  its  practical  application  in  the  wire  mill  covers  principally 
the  medium-  and  higher-carbon  steels.  The  structure  of  wire  rods 
with  regard  to  size  of  grain  is  dependent  upon  the  temperature  at 
which  the  rods  are  finished  in  the  hot  rolling  mill  and  upon  the  rate 
of  cooling  through  the  critical  temperature  of  the  steel.  In  steel  of 
low  carbon  this  is  not  of  as  much  importance  as  in  the  higher-carbon 
steels,  for  the  reason  that  the  ordinary  finishing  temperature  varia- 
tions of  good  rolling-mill  practice  have  less  effect  on  grain  structure 
of  soft  rods,  and  therefore  less  effect  on  their  physical  properties. 
In  higher-carbon  steels  a  fine  grain  is  important,  for  it  is  this  struc- 
ture that  makes  for  such  steels  their  field  of  usefulness,  where  high 
strength,  high  elastic  limit  and  toughness  are  required. 

Theoretically,  the  ideal  structure  would  be  obtained  if  the  entire 
rod  could  be  finished  at  about  the  critical  temperature.  But  this 
is,  of  course,  impracticable,  for  the  reason  that  it  is  impossible 
to  regulate  the  finishing  temperatures  so  closely,  and  for  the  addi- 
tional reason  that  there  is,  necessarily,  particularly  in  rolling  very 
long  lengths  of  very  small  sections,  a  marked  difference  between  the 
finishing  temperatures  of  the  first  and  last  end  of  a  rod.  The  higher 
the  finishing  temperatures  above  the  critical  range  the  coarser  the 
grain,  and  the  coarser  the  grain  the  more  does  the  steel  lack  the 
qualities  that  give  it  value.  In  order  to  destroy  the  coarse  or  uneven 
structure  that  may  be  created  as  just  described,  it  is  necessary  to 
anneal  the  steel  by  heating  it  just  above  its  critical  temperature  and 
slowly  cooling  it  down. 

The  effect  of  overheating  in  coarsening  the  grain  structure  of 
a  0.45  per  cent,  carbon  steel  and  the  refining  influence  of  this  type 
of  annealing  is  shown  in  Figs.  211  and  212. 

The  third  and  last  class  of  annealing  to  be  described— that  to 
obtain  definite  structure — is  one  of  comparatively  recent  develop- 
ment in  the  steel-wire  industry  and  one  which  promises  to  be  of  con- 
siderable  value.  Annealing  of  this  type  is  applied  principally  to 
the  higher  carbon  wires.  Since  the  structure  of  such  wires  can  be 
varied  considerably  within  a  small  range  of  annealing  temperatures, 
it  covers  specific  products  and  not  general  classes,  as  would  be  the 
case  in  regard  to  the  two  previously  described  types  of  annealing, 
Figs.  213  and  214  illustrate  excellently  this  special  type  of  annealing. 
These  photomicrographs  show  the  structures  of  two  annealed  pieces 
of  the  same  coil  of  high-carbon  wire,  in  which  the  annealing  temper- 
ature of  the  one  specimen  was  130fr°  F.,  and  of  the  other  1250°  F. 


402  STEEL  AND   ITS  HEAT  TREATMENT 

It  is  impossible  to  identify  the  structure  by  a  simple  observation  of 
the  fracture,  which  is  the  ordinary  rough-and-ready  method;  nor 
is  it  possible  to  regulate  annealing  temperatures  so  closely  without 
the  use  of  pyrometers. 

In  passing  to  the  next  great  class  of  heat  treatment  applied  to 
steel  wire,  patenting,  it  is  interesting  to  note  that  we  likewise  pass 
to  another  class  of  wire  as  regards  grading  by  carbon  content.  It 
naturally  covers  the  medium-carbon  steels,  being  employed  chiefly  on 
carbons  between  0.35  and  0.85  per  cent.  In  the  medium-carbon 
steel  wires  strength  and  toughness  are  required  for  both  process  and 
finished  wire.  Patenting  makes  possible  this  combination  of  strength 
and  toughness,  and  to  this  process  is  due  in  large  measure  a  broad 
field  of  application  for  steel  wire. 

The  high  strength  and  toughness  of  patented  wire  are  due  to 
its  carbon  condition  and  to  its  peculiar  structure.  The  first  step 
in  the  patenting  process  is  to  heat  the  wire  to  a  temperature  above 
its  critical  range.  The  degree  of  heating  is  regulated  according  to  the 
carbon  content  of  the  steel,  the  size  of  rod  or  wire,  and  the  time  the 
material  is  subjected  to  the  heat.  After  sufficient  heating,  the  next 
step  is  to  cool  the  material  rapidly  below  its  critical  range,  the 
structure  obtained  depending  upon  the  rate  of  cooling.  In  practice, 
patenting  is  usually  conducted  as  a  continuous  operation,  the  wire 
being  led  through  the  heated  tubes  of  a  furnace  and  cooled  by  being 
brought  into  the  air  or  into  a  bath  of  molten  lead  comparatively 
cool  but  seldom  under  700°  F. 

A  better  understanding  of  the  structure  of  a  patented  wire  may 
be  had  by  a  comparison  of  the  structure  obtained  by  slow  and  by 
rapid  cooling.  If  the  steel  after  being  heated  is  allowed  to  cool 
slowly  through  the  critical  temperature  range,  the  homogeneous  pre- 
existing solid  solution  of  iron  and  iron  carbide  separates  into  a  hetero- 
geneous mixture  of  two  constituents,  resulting  in  the  plate-like  struc- 
ture called  "  pear  lite."  In  a  patented  wire,  part  of  the  carbide  of 
iron  is  in  solid  solution  and  the  remainder,  while  not  in  solid  solution, 
has  not  had  time  to  form  into  plates.  The  difference  in  structure 
between  slow  and  rapid  cooling  is  seen  in  Figs.  213  and  215.  The 
photomicrograph  of  the  patented  wire  shows,  as  a  result  of  the  rapid 
cooling,  a  structure  that  might  be  termed  nondescript.  Metallo- 
graphists  will  recognize  the  structure  as  "  sorbite,"  which,  in  the 
cooling  of  the  higher-carbon  steels  from  above  the  critical  tempera- 
ture, is  that  stage  of  transition  just  preceding  the  pearlitic,  the  final 
condition  of  annealed  steel  as  shown  in  Fig.  213.  The  patented 


MISCELLANEOUS  TREATMENTS 


403 


FIG.  210.— Steel  Wire  (0.08  Carbon) 
Hard  Drawn  and  then  Annealed 
below  the  Critical  Temperature. 
(Tinsley.) 


FIG.  211.— Steel  (0.45  Carbon)  Over 
heated.     (Tinsley.) 


FIG.  212.— Steel    (0.45  Carbon) 
Annealed.     (Tinsley.) 


FIG.  213.— Annealed  (0.85  Carbon) 
Steel.     (Tinsley.) 


FIG.  214.— Specially  Annealed  (0.85 
Carbon)  Steel  for  Globular 
Structure.  (Tinsley.) 


FIG.  215.— Patented  (0.85  Carbon) 
Steel.     (Tinsley.) 


404 


STEEL  AND   ITS  HEAT  TREATMENT 


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MISCELLANEOUS  TREATMENTS  405 

wire,  therefore,  represents  an  unsegregated  condition  as  against  the 
segregated  or  coarsely  laminated  structure  of  annealed  wire.  The 
high  tensile  strength  of  patented  wire  is  due  to  the  amount  of  carbon 
in  solution,  and  its  toughness  to  the  fineness  of  the  grain  structure. 

Patenting  serves  two  important  functions  in  the  wire  business: 
1.  In  the  process  of  manufacture,  the  removal  of  the  effects  of  cold 
work,  such  as  drawing.  2.  In  the  finished  wire  to  give,  in  conjunc- 
tion with  cold  drawing,  the  required  combination  of  strength  and 
toughness.  Strictly  speaking,  patenting  is  not  necessary  simply 
to  relieve  strain,  for  annealing  would  serve  that  purpose,  but  the 
structure  obtained  by  patenting  permits  much  further  cold  drawing 
than  does  the  structure  obtained  by  annealing.  This  is  due  primarily 
to  the  increased  ductility  and  toughness  of  the  patented  wire.  The 
effect  of  patenting  as  just  described  is  shown  in  Fig.  217. 

In  wire  making,  hardening  and  tempering  should  be  conducted 
usually  as  a  continuous  process.  In  the  making  of  tempered  wire 
the  material  is  first  run  through  the  heated  tubes  of  a  furnace, 
then  quenched  quickly  in  a  bath  of  oil  or  water,  then  run  into  the 
tempering  bath  of,  say,  molten  lead,  each  wire  being  in  continuous 
motion  from  the  time  it  enters  the  heating  furnace  until  it  is  wound 
on  a  reel.  Hardening  and  tempering  apply  to  the  higher  carbon  steel 
wires — those  in  which  the  carbon  range  is  from  0.65  per  cent=  to 
1.00  per  cent.  With  varying  tempering  temperatures  between  500° 
and  1100°  F.,  the  tensile  strength  runs  from  about  340,000  Ibs.  per 
square  inch  to  150,000  Ibs.  per  square  inch.  At  the  lower  temper- 
ature the  decrease  in  tensile  strength  is,  as  we  should  expect,  much 
greater  per  100°  F.  range  than  at  the  higher  temperatures.  From 
500°  to  600°  F.  there  is  a  drop  of  60,000  Ibs.  per  square  inch,  while 
between  1000°  F.  and  1100°  F.  the  drop  in  tensile  strength  amounts 
to  only  about  10,000  Ibs.  per  square  inch. 

FORGING 

No  small  percentage  of  the  difficulty  encountered  in  heat-treat- 
ment operations  is  due  to  improper  forging  methods,  and  ofttimes 
the  heat-treatment  operation  is  nothing  more  than  a  useless  effort 
or  attempt  to  get  something  out  of  a  forged  piece  of  steel  that  is  not 
actually  in  it.  Thus,  the  steel  man  is  often  blamed  for  the  absence 
of  quality  in  his  steel  that  he  actually  put  in  it;  and  the  heat-treat- 
ment man  is  blamed  for  his  lack  of  ability  to  locate  such  qualities, 
which  he  properly  assumes  to  exist,  but  which,  nevertheless,  the  forge 
man  took  out  by  poor  heating,  unknown  to  himself  or  the  other  two. 


406  STEEL  AND   ITS   HEAT  TREATMENT 

The  strongest  language  that  could  be  employed  in  an  attempt 
to  describe  the  general  average  heat-treatment  equipment,  the 
methods  of  heating,  and  personnel,  as  they  are  actually  known  to 
exist,  would  be  altogether  too  mild  and  ineffective  for  a  proper 
description  of  the  heating  methods  and  equipment  in  the  majority 
of  forge  shops  in  the  country.  As  in  the  case  of  machine  work,  the 
design  of  the  hammers,  presses  and  other  machine  equipment  has 
made  rapid  strides  forward,  but  the  two  most  important  factors 
of  the  operation  from  the  metallurgical  end — namely,  the  man  and 
the  furnace — have  either  stood  still  or  gone  backwards.  Many 
well-informed  and  experienced  men  claim  that  the  caliber  of  forge 
men  to-day  is  not  what  it  was  years  ago,  and  that  better  quality  of 
work  was  produced  with  the  old-fashioned  coal  or  coke  furnaces, 
though  at  a  higher  cost,  than  at  present  with  furnaces  burning  oil 
or  gas.  There  appears  to  be  something  in  this  statement,  particu- 
larly in  view  of  the  high  quality  work  turned  out  in  Europe,  where 
the  use  of  high-speed  machines,  oil  or  gas  fuel,  and  efficiency  pro- 
duction methods,  are  not  as  prevalent  as  here.  If  such  a  difference 
actually  exists,  it  can  invariably  be  traced  to  the  personnel  of  the 
plant,  because,  as  in  most  operations  involving  the  skill  of  the  oper- 
ator as  against  the  fixed  movement  of  a  machine,  quality  reflects 
the  man  and  his  knowledge  of  the  work.  But  even  so,  we  can  and 
should  be  able  to  do  better  with  fuel  so  closely  linked  with  uni- 
formity of  temperature,  steadiness  of  operation,  and  ease  of  control. 
If  we  do  not,  then  it  is  up  to  the  man  or  the  furnace  and  not  to  the 
hammer  or  the  fuel,  which  is  in  itself  a  good  argument  for  improve- 
ment of  the  heating  and  human  equations  in  the  operation. 

Two  of  the  weak  links  in  forging  practice,  from  the  metallurg- 
ical end,  are  the  lack  of  uniformity  and  temperature  of  the  heats 
and  the  method  of  handling  stock  in  and  out  of  the  furnaces. 

As  a  rule,  the  heats  are  altogether  too  high,  with  the  result  that, 
while  the  surface  is  apparently  hot,  there  may  be  actually  a  "  bone  " 
on  the  inside.  It  is  common  practice  to  see  a  bar  drawn  from  a 
furnace  that  will  actually  drip,  and  yet  when  placed  under  the  ham- 
mer there  will  be  indications  of  lack  of  heating  on  the  inside.  It  is 
the  inside  of  the  bar  that  determines  the  physical  properties  of  the 
final  forging  and  not  the  outside;  and  there  is  nothing  gained  in 
these  quick  "  wash  "  or  surface  heats.  Slow,  soft,  soaking  heats, 
affording  plenty  of  time  to  heat  up,  are  more  desirable  than  the 
higher  quick  heats.  The  idea  should  be  to  maintain  the  temperature 
of  the  furnace  as  near  as  possible  to  that  actually  required  to  soften 


MISCELLANEOUS  TREATMENTS  407 

the  steel  to  the  extent  necessary  for  its  proper  shaping,  and  to  give 
it  plenty  of  time  in  the  furnace  thoroughly  to  soak  at  this  temper- 
ature without  overheating  or  oxidizing  the  outside.  The  fire  should 
be  soft  and  a  little  high  in  carbon,  in  order  to  reduce  oxidation.  The 
modern  alloy  steels  do  not  require  high,  sharp,  dripping  heats, 
and  the  proper  handling  of  them  demands  the  slow,  soft,  non- 
oxidizing  heats  above  referred  to. 

The  general  design  of  forge  furnaces  is  far  below  the  standard 
of  heat-treating  furnaces  and  is  a  point  usually  left  to  the  forge  man 
or  to  a  bricklayer.  It  is  common  practice  to  see  furnaces  hot  on 
one  side  and  cold  on  the  other.  Also,  to  hear  complaints  of  lack  of 
ability  to  heat  steel  properly  in  a  furnace  in  which  the  burners  blast 
directly  against  the  stock,  which  naturally  keeps  the  stock  nearest 
the  burner  cool  and  heats  the  pieces  farther  away.  There  are 
hundreds  of  such  designs  in  use  that  have  been  turned  out  by  furnace 
builders  who  ought  to  know  better. 


CHAPTER   XVIII 
PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS 

PYROMETERS 1 

Pyrometers  in  General. — The  pyrometer  has  played  a  basic  part 
in  the  development  of  intelligent  heat  treatment.  In  hardening 
rooms  where  pyrometers  are  not  used,  a  discussion  of  any  temper- 
ature treatment  and  instructions  are  given  as  the  instructions 
must  have  been  given  in  the  Tower  of  Babel.  There  is  no  dis- 
tinction or  mutual  understanding  of  terms,  and  until  a  pyrometer 
— and  an  accurate  one — is  in  a  hardening  room,  it  is  not  possible 
for  those  interested  in  the  heat  treatment  in  that  room  to  talk 
to  each  other  in  a  mutually  intelligible  way.  Of  course,  where  one 
old  hardener  has  been  in  charge  for  twenty  years  and  the  manage- 
ment decides  to  take  a  chance  on  his  staying  with  them  and  living 
for  another  twenty  years,  it  may  be  all  right  to  have  everything 
locked  up  in  his  head;  but  where  matters  are  more  extensively  and 
more  modernly  conducted,  it  is  necessary  to  have  some  language  in 
which  people  can  talk;  and  the  pyrometer,  by  virtue  of  its  tempera- 
ture scale,  which  is  a  conventional  scale  of  denned  terms,  affords 
the  means  of  communication  in  a  language  that  is  mutually  under- 
stood. In  the  same  way  it  permits  records  to  be  kept  for  future 
reference.  Where  this  is  not  done,  men  will  be  found  trying  to 
remember  the  heats  at  which  they  treated  this,  that  or  the  other 
lot  of  steel;  they  cannot  remember,  and  they  are  sure  to  get  into 
trouble  if  they  try  to.  The  pyrometer  has  changed  barbarian 
methods  into  civilized  methods  in  a  hardening  room. 

There  is  need  for  a  greatly  extended  use  of  pyrometers  of  the 
best  possible  grade,  but  more  especially  for  an  intelligent  use  of  them 
that  will  in  some  measure  compensate  for  the  skill  in  producing  them 

1  It  is  the  aim  of  this  section  to  deal  more  with  the  rational  use  of  pyrometers 
rather  than  with  a  detailed  explanation  of  the  theory  and  construction  of  the 
numerous  instruments  in  commercial  use  for  heat  measurement.  For  a  fuller 
explanation  of  the  latter  subject  than  is  subsequently  given,  the  reader  is  referred 
to  standard  reference  books  on  the  subject. 

408 


PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS    409 

and  the  money  involved  in  their  installation.  The  pyrometer  is 
not  all-sufficient,  nor  it  is  the  cure-all  for  the  troubles  of  a  hardening 
plant.  There  should  be  an  education  of  the  man  to  look  upon 
pyrometers  as  gauges  and  indicators  of  the  existence  of  energy, 
and  as  an  aid  to  him  in  executing  his  work  and  not  as  a  means  of 
releasing  him  from  responsibility  accompanying  the  exercise  of 
judgment. 

The  pyrometer  has  been  of  inestimable  value  in  affording  a 
means  to  check  temperature,  but — and  aside  from  the  correlation 
of  results — its  efficiency  ends  largely  with  that  indication.  The 
uniformity  of  heated  product,  however,  depends  upon  the  manner 
of  applying  the  heat,  which — with  the  method  and  cost  of  operation 
— is  primarily  a  function  of  furnace  design.  It  is  possible  to  indicate 
a  uniform  temperature  and  yet  not  produce  a  uniformly  heated  prod- 
uct; and  unless  the  heat  is  uniformly  applied  to  the  stock  at  the 
temperature  indicated,  then  a  uniform  pyrometer  reading  is  mis- 
leading and  inconclusive.  Thus  an  elaborate  pyrometer  system, 
with  means  for  signaling  variations  in  temperature  and  of  record- 
ing these  variations,  is  not  conclusive  evidence  of  accurate  heating. 
The  development  toward  better  and  cheaper  results  will  be  brought 
about  by  improved  heating  methods,  even  though  the  temperature 
recorded  from  any  one  point  in  a  furnace  chamber  may  be  the  same 
as  that  indicated  from  a  similar  point  of  another  furnace  less  effici- 
ently designed. 

The  time  element  is  linked  inseparably  with  all  heating  opera- 
tions. A  piece  of  steel  can  absorb  heat  only  so  fast  and  no  faster. 
Only  by  operating  the  furnace  so  that  the  maximum  temperature 
is  maintained  for  the  length  of  time  necessary  uniformly  to  heat 
the  steel  throughout  to  that  temperature,  is  it  possible  to  produce 
the  best  results.  In  other  words,  the  composition  and  the  mass 
of  the  steel  must  be  correlated  with  the  time  element.  First  deter- 
mine the  length  of  time  necessary,  under  standard  furnace  conditions, 
to  produce  the  necessary  results;  then  regulate  the  furnace  by  the 
aid  of  the  pyrometer;  and  finally,  place  a  clock  beside  the  instru- 
ment and  work  the  two  together.  The  sooner  the  average  heat- 
treatment  man  (and  his  superiors,  for  that  matter)  can  be  brought 
to  realize  that  a  pyrometer  is  almost  valueless  without  the  use  of  a 
time  clock  and  common-sense  observation  of  furnace  conditions,  the 
better. 

Thermo-Couples. — For  the  usual  operations  in  heat-treatment 
work  involving  temperatures  of  over  600°  or  700°  F,,  the  thermo- 


410  STEEL  AND   ITS  HEAT  TREATMENT 

couple  system  is  the  most  used.  The  principles  upon  which  its  use 
depends  are  simple.  Expressed  briefly,  if  the  ends  of  two  pieces  of 
dissimilar  metals  (usually  as  wires)  are  joined  together  and  one 
of  the  junctions  (the  "  hot  end  ")  is  heated,  the  other  junction  (the 
"  cold  end  ")  being  held  at  a  constant  temperature,  a  feeble  electric 
current  is  generated  in  the  circuit.  This  electromotive  force,  aside 
from  being  dependent  upon  the  nature  of  the  couple,  is,  for  the 
thermo-couples  in  practical  use,  dependent  upon  the  difference  in 
temperature  between  the  hot  and  cold  ends. 

In  regard  to  thermo-couples,  standard  base-metal  compositions 
will  generally  give  satisfaction  between  600°  or  700°  F.  and  1800°  F.; 
while  above  1800°  F.  couples  of  platinum  and  platinum-rhodium 
should  be  used.  All  base-metal  couples  should  be  readily  replace- 
able, and,  more  emphatically,  interchangeable.  All  couples  should 
be  suitably  protected  with  iron  pipes  from  oxidation  and  rough 
handling. 

Position  of  the  Thermo-Couple. — The  fact  that  a  pyrometer 
may  show  that  some  particular  portion  of  the  heating  zone  is  at  the 
proper  temperature  is  no  proof  that  the  steel  is  also  at  that  temper- 
ature. The  hot  end  of  the  couple  may  be  so  placed  that  it  must 
inevitably  be  hotter  than  the  hearth  of  the  furnace,  or  hotter  than 
any  material  placed  on  the  hearth.  This  will  be  true  if  the  end  of 
the  couple  is  exposed  to  the  direct  heat  of  the  flame.  It  might 
therefore  be  concluded  that  the  tip  should  be  as  near  the  work  as 
is  possible,  so  that  both  may  attain  the  same  temperature — and 
which  is  without  doubt  advisable  in  many  instances.  On  the  other 
hand,  it  has  been  noted  in  some  cases  in  which  the  couples  have 
been  placed  close  to  the  work  that  the  readings  are  not  in  accord 
with  the  temperature  of  the  steel  because  the  couples,  being  of 
smaller  mass,  take  up  readily  the  high  peak  of  the  flame  tempera- 
ture. There  are  certain  instances  where  it  has  been  found  by 
experience  desirable  to  locate  the  tip  of  the  couple  in  a  recess  in 
the  furnace  wall  where  it  was  out  of  the  course  of  the  flame  and 
thus  dependent  for  its  temperature  upon  radiation  from  the  main 
body  of  the  furnace  lining  and  radiation  from  the  work;  under 
some  circumstances  such  a  position  is  preferable. 

Millivoltmeter  vs.  Potentiometer. — By  inserting  into  the  thermo- 
couple circuit,  at  the  cold  junction,  a  suitable  device  for  measuring 
the  electromotive  force,  a  reading  may  be  obtained  in  millivolts; 
or,  by  suitable  calibration,  a  reading  directly  in  terms  of  temper- 
ature. This  indicating  instrument  (the  pyrometer)  may  be  of 


PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS    411 

the  galvanometer  or  millivoltmeter  type,  or  of  the  potentiometer 
type. 

The  potentiometer  in  theory  bears  much  the  same  relation  to  the 
millivoltmeter  that  the  balance-arm  scales  bears  to  the  spring  scales. 
The  constancy  of  both  the  spring  scales  and  the  millivoltmeter  is 
entirely  dependent  upon  the  constancy  of  springs  or  of  suspensions, 
and  upon  the  absence  of  friction.  The  constancy  of  the  potentiom- 
eter and  of  the  balance-arm  scales  is  dependent  only  upon  the  con- 
stancy of  standard  weights  in  one  case  and  a  standard  electromotive 
force  in  the  other.  Standard  weights  are  added  to  or  removed 
from  balance  scales  until  a  balance  between  known  and  unknown 
is  obtained.  Similarly  in  the  potentiometer  type  varying  fractions 
of  a  known  and  presumably  standard  electromotive  force  are  opposed 
to  the  electromotive  force  of  the  thermo-couple  until  it  is  balanced 
just  as  a  standard  weight  is  moved  along  a  scale  arm  for  balance. 

This  is  the  potentiometer  not  as  it  is,  but  as  we  would  like  to  have 
it.  The  standard  cell  will  not  stay  standard  if  any  current  is  drawn 
from  it  and,  consequently,  the  e.m.f.  of  the  standard  cell  is  not 
opposed  to  the  e.m.f.  of  the  couple  in  potentiometers  as  made  for 
any  ordinary  use.  Another  cell  or  battery  is  brought  into  use  and 
the  e.m.f.  of  that  is  opposed  to  the  e.m.f.  of  the  couple.  Now 
this  secondary  cell  varies  in  e.m.f.  from  week  to  week  and  day  to 
day,  and  even  hour  to  hour  under  use,  and  it  is  necessary  contin- 
ually to  check  this  service  cell  against  a  standard  cell  and  then  to 
adjust  for  the  differences  that  are  creeping  in  all  the  time.  The 
balance  scales,  therefore,  instead  of  being  operative  with  standard 
weights,  have  a  sort  of  beaker  of  boiling  water  as  the  weight,  which 
is  continually  boiling  to  less  mass  and  which  has  to  be  filled  up  or 
adjusted  every  few  minutes  by  comparing  it  with  a  standard  weight, 
for  the  standard  weight  itself  is  not  trusted  on  the  scales  nor  is  there 
any  other  weight,  i.e.,  battery,  that  can  be  trusted  on  the  scales  that 
will  not  vary. 

Selection  of  Equipment. — The  selection  of  one  type  or  the  other 
is  largely  a  matter  for  economic  and  technical  consideration.  In  a 
word,  the  purchaser  should  consider  the  relation  existing  between 
(1)  accuracy,  sensitiveness  and  constancy,  (2)  ease  of  reading,  and 
(3)  the  cost — both  initial  and  of  up-keep.  There  is  also  a  psycho- 
logical consideration  that  goes  hand  in  hand  with  the  above  consider- 
ations and  which  should  not  be  lost  sight  of:  the  millivoltmeter 
is  a  direct-reading  instrument,  which  means  that  it  is  easy  to  read; 
the  potentiometer  requires  a  fair  amount  of  manipulation  and  is 


412 


STEEL  AND  ITS  HEAT  TREATMENT 


somewhat  less  easy  to  read.  The  question  then  is:  Which  instru- 
ment will  the  average  furnace  man  read  more  frequently?  No 
matter  how  accurate  a  pyrometer  may  be,  its  value  is  only  in  the 
use  made  of  it. 

Cold-end  Temperature. — The  cold-end  temperature  is  a  source 
of  prolific  error  in  some  pyrometer  installations.  It  should  be 
remembered  that  all  instruments  are  calibrated  for  a  definite  cold- 
end  temperature,  usually  75°  F.  If  the  cold  end  is  in  a  position  such 
that  it  receives  the  direct  or  radiating  heat  from  the  furnace,  and 


Copper  LeaJs^ 


groxm  A- re  n  .i  pevafiii- e 

•*.V-  :•; : ;  '•/  r:'v&2  • 


FIG.  218. — Compensating  Cold  End  Temperatures  with  Auxiliary  Couple. 
(Wilson-Maeulen  Co.) 

therefore  varies  in  temperature,  the  indicated  temperature  at  the 
instrument  will  be  incorrect.  For  this  reason  the  cold  end  should 
always  be  kept  cool,  and  at  as  near  a  constant  temperature  as  is 
possible.  This  compensation  may  be  accomplished  by  having  the 
cold  end  as  near  the  ground  as  possible;  or  by  letting  a  small  stream 
of  cold  water  flow  over  the  cold  ends;  or  by  connecting  an  auxiliary 
couple  of  the  same  electromotive  force  as  the  furnace  couple  in  oppo- 
sition to -the  couple  in  the  furnace,  and  running  the  auxiliary  couple 
to  an  underground  point  at  the  bottom  of  a  pipe  driven  a  few  feet 
into  the  earth  as  shown  in  Fig.  218.  The  potentiometer  type 
equipment  frequently  carries  the  cold  end  directly  to  the  instrument, 


PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS    413 

entirely  eliminating  the  effect  of  fluctuating  temperatures  near  the 
furnace. 

Pyrometer  Standardization. — One  of  the  most  important  points  in 
connection  with  pyrometers  is  the  necessity  for  frequent  and  regular 
calibration  of  the  thermo-couples.  All  base-metal  couples  should 
be  standardized  at  least  once  a  week,  and  oftener  if  possible.  Fur- 
ther, new  couples  should  always  be  standardized  before  use,  since 
errors  may  frequently  be  found  even  in  supposedly  correct  new 
couples. 

There  are  two  general  methods  for  standardization  or  calibration 
of  thermo-couples:  (1)  Checking  against  the  melting-  or  freezing- 
points  of  known  salts  or  metals,  and  (2)  checking  against  a  standard 
millivolt  meter  or  pyrometer. 

Standardization  with  Common  Salt. — An  easy  and  convenient 
method  1  for  standardization  and  not  necessitating  the  use  of  an 
expensive  laboratory  equipment  is  that  based  upon  determining 
the  melting-point  of  common  table  salt  (sodium  chloride).  While 
theoretically  salt  that  is  chemically  pure  should  be  used  (and  indeed 
this  is  neither  expensive  nor  difficult  to  procure),  commercial  accu- 
racy may  be  obtained  by  using  common  table-salt  such  as  is  sold  by 
every  grocer.  The  salt  is  melted  in  a  clean  crucible  of  fire-clay,  iron 
or  nickel,  either  in  a  furnace  or  over  a  forge-fire,  and  then  further 
heated  until  a  temperature  of  about  1600°  to  1650°  F.  is  attained. 
It  is  essential  that  this  crucible  be  clean,  because  a  slight  admixture 
of  a  foreign  substance  might  noticeably  change  the  melting-point. 
The  thermo-couple  to  be  calibrated  is  then  removed  from  its  protect- 
ing tube  and  its  hot  end  is  immersed  in  the  salt  bath.  When  this 
end  has  reached  the  temperature  of  the  bath,  the  crucible  is  removed 
from  the  source  of  heat  and  allowed  to  cool,  and  cooling  readings  are 
then  taken  every  ten  seconds  on  the  millivoltmeter  or  pyrometer. 
A  curve  is  then  plotted  by  using  time  and  temperature  as  co-ordinates, 
and  the  temperature  of  the  freezing-point  of  salt,  as  indicated  by 
this  particular  thermo-couple,  is  noted,  i.e.,  at  the  point  where  the 
temperature  of  the  bath  remains  temporarily  constant  while  the 
salt  is  freezing.  The  length  of  time  during  which  the  temperature 
is  stationary  depends  on  the  size  of  the  bath  and  the  rate  of  cooling, 
and  is  hot  a  factor  in  the  calibration.  The  melting-point  of  salt  is 
1472°  F.  and  the  needed  correction  for  the  instrument  under  obser- 
vation can  be  readily  applied.  The  curves  in  Figs.  219  and  220  illus- 
trate the  calibration  of  a  correct  and  incorrect  pyrometer. 

1  Carpenter  Steel  Co. 


414 


STEEL  AND  ITS  HEAT  TREATMENT 


180 
160 
140 
120 
100 
80 
60 
40 
20 
0 

/ 

/\ 

/ 

/ 

/ 

/ 

\ 

/ 

/ 

1 

I 

I 

J 

/ 

7 

s" 

r 

^ 

'-""" 

-' 

x 

s* 

1050°                  1600°                  1550°                   1500°                  1450° 

Degrees  Fahrenheit 

FIG.  219. — Diagram  Showing  the  Calibration  of  a  Pyrometer  which  Reads  45°  F. 
Too  High.     (Carpenter  Steel  Co.) 


180 
100 
140 
120 
100 
SO 
GO 
40 
20 

0 

] 

/ 

J\ 

\ 

f 

1 

• 

/ 

/ 

^ 

** 

^ 

<< 

"*'  • 

^ 

•i*1 

^ 

x* 

1650°                   1600°                    1550°                   1500°                  1450° 
.Degrees  Fahrenheit 

FIG.  220. — Diagram  Showing  the  Calibration  of  a  Pyrometer  which  is  Correct. 

(Carpenter  Steel  Co,) 


PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS    415 

It  should  not  be  understood  from  the  above,  however,  that  the 
salt-bath  calibration  cannot  be  made  without  platting  a  curve :  in 
actual  practice  at  least  a  hundred  tests  are  made  without  platting 
any  curve  to  one  in  which  it  is  done.  The  observer,  if  awake,  may 
reasonably  be  expected  to  have  sufficient  appreciation  of  the  lapse 
of  time  definitely  to  observe  the  temperature  at  which  the  falling 
pointer  of  the  instrument  halts.  The  gradual  dropping  of  the  pointer 
before  freezing,  unless  there  is  a  large  mass  of  salt,  takes  place  rapidly 
enough  for  one  to  be  sure  that  the  temperature  is  constantly  falling 
and  the  long  period  of  rest  during  freezing  is  quite  definite.  The 
procedure  of  detecting  the  solidification  point  of  the  salt  by  the 
hesitation  of  the  pointer  without  platting  any  curve  is  suggested 
because  of  its  simplicity. 

Complete  Calibration  of  Pyrometers. — For  the  complete  calibra- 
tion of  a  thermo-couple  of  unknown  electromotive  force,  the  new 
couple  may  be  checked  against  a  standard  instrument,  placing  the 
two  bare  couples  side  by  side  in  a  suitable  tube  and  taking  frequent 
readings  over  the  range  of  temperatures  desired. 

If  only  one  instrument,  such  as  a  millivoltmeter,  is  available, 
and  there  is  no  standard  couple  at  hand,  the  new  couple  may  be 
calibrated  over  a  wide  range  of  temperatures  by  the  use  of  the  follow- 
ing standards ; 

Water,  Boiling-point 212°  F. 

Tin,  under  charcoal,  Freezing-point 450 

Lead,  under  charcoal,  Freezing-point 621 

Zinc,  under  charcoal,  Freezing-point 786 

Sulphur,  Boiling-point 832 

Aluminum,  under  charcoal,  Freezing-point 1216 

Sodium  chloride,  Freezing-point 1474 

Potassium  sulphate,  Freezing-point 1958 

A  good  practice  is  to  make  one  pyrometer  a  standard;  calibrate 
it  frequently  by  the  melting-point-of-salt  method,  and  each  morning 
check  up  every  pyrometer  in  the  works  with  the  standard,  making 
the  riecessary  corrections  to  be  used  for  the  day's  work.  By  pur- 
suing this  course  systematically,  the  improved  quality  of  the  product 
will  much  more  than  compensate  for  the  extra  work. 

Central  Switch-boards. — For  plants  in  which  there  are  a  number 
of  thermo-couples,  one  indicating  instrument  with  a  central  switch- 
board may  be  used.  As  many  as  sixteen  couples  may  be  wired  to 
one  selective  switch,  the  maximum  number  simply  depending  upon 
the  elasticity  of  the  system  and  the  convenience  of  the  operator. 


416 


STEEL  AND   ITS  HEAT  TREATMENT 


A  wiring  diagram  for  such  an  installation  is  shown  in  Fig.  221.  By 
throwing  the  switch  from  one  contact  to  another  the  connection 
is  made  with  each  individual  furnace.  For  large  heat-treatment 
plants  the  time  of  one  man  is  generally  taken  in  attending  to  the 
system,  he  signaling  the  individual  operators  by  means  of  lights 
and  belts  the  relative  temperatures  in  the  furnace.  We  have 
previously  commented  upon  such  systems. 

The  Central  System. — The  Chalmers  Motor  Company  operate 
their  system,1  having  two  central  switch-boards  with  sixteen  furnaces 
on  a  switch,  as  follows; 


Couple  8 


Couple  I  , 


FIG.  221. — Wiring  Diagram — Pyrometer  and  Selective  Switch.  Showing  Four 
Couples  Connected  with  the  Switch,  Openings  for  Four  More. 
(Hoskins  Mfg.  Co.) 

"  We  regulate  the  heat  of  the  furnaces  by  a  series  of  lights — 
each  furnace  having  over  it  a  red,  blue  and  green  light.  These  are 
used  as  follows:  We  will  say  that  the  temperature  of  an  empty 
furnace  which  we  are  about  to  use  is  1600°  F.  The  loading  of  the 
furnace  with  forgings  necessarily  reduces  the  heat  by  radiation  any- 
where from  100°  to  250°,  depending  upon  the  number  of  pieces  put 
in  the  furnace.  When  we  commence  to  bring  the  heat  up  again  to 
the  proper  place  and  it  gets  to  about  1575°,  the  man  at  the  switch- 
board throws  on  a  blue  light,  which  means  to  the  furnace  operator 
that  the  heat  is  still  considerably  too  low.  When  the  temperature 
reaches  about  1590°  the  blue  and  green  light  is  turned  on,  which 
signifies  to  the  operator  that  the  furnace  is  still  not  quite  hot  enough. 
1  Personal  Correspondence. 


PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS    417 

When  the  1600°  point  is  reached  the  green  light  is  turned  on;  this 
is  the  O.  K.  light  and  means  that  the  temperature  is  correct.  The 
steel  is  then  allowed  to  soak  at  this  temperature  for  the  time  neces- 
sary to  affect  the  whole  mass.  If  the  heat  during  the  operation 
gets  too  high  we  use  signals  in  an  inverse  manner,  the  red  and  green 
lights  being  thrown  on.  If  it  shows  a  dangerous  rise  in  temperature 
the  red  light  is  thrown  on.  All  of  these  lights  are  accompanied  by 
the  ringing  of  a  loud  bell  in  the  heat-treating  department,  which 
automatically  attracts  the  attention  of  the  man  operating  the 
furnaces,  who  at  once  inspect  their  individual  furnace  lights  to  see 
if  their  temperature  is  correct." 

DETERMINATION    OF   THE    CRITICAL   RANGES 

Critical  Ranges. — The  practical  importance  of  knowing  the 
exact  location  of  the  critical  ranges  of  steel  to  be  treated  is  obvious. 
Their  determination  by  means  of  pyrometers  is  based  upon  the  fact 
that  the  changes  taking  place  in  the  steel  at  those  temperatures 
involve  an  absorption  of  heating  during  heating  (the  decalescent 
points)  and  a  giving  out  of  heat  on  passing  through  these  ranges  on 
cooling  (the  recalescent  points). 

Decalescent  vs.  Recalescent  Points. — Before  discussing  methods, 
it  should  be  stated  that,  for  the  majority  of  heat-treatment  work, 
it  is  more  important  to  know  the  location  of  the  decalescent  points 
than  that  of  the  recalescent  points.  This  is  for  several  reasons.  To 
effect  a  complete  change  of  the  original  structure  of  the  steel,  it  must 
be  at  least  heated  slightly  beyond  the  Ac3  range,  regardless  of  the 
position  of  the  Ar3  range.  If  the  steel  were  to  be  heated  only  to  the 
Ar3  range,  a  complete  change  in  structure  cannot  take  place,  because 
the  Ar3  range  is  always  below  the  temperature  of  the  Ac3  range. 
Further,  the  position  of  the  Ar  ranges  is,  experimentally  at  least, 
dependent  upon  the  maximum  temperature  to  which  the  steel  is 
heated,  upon  the  length  of  heating  at  that  temperature,  and  upon 
the  rate  of  cooling  from  that  temperature. 

It  should  also  be  again  stated  that  the  determination  of  the 
upper  critical  range  is  of  more  importance  than  that  of  the  lower 
critical  range  (Al),  since  the  majority  of  hardening  and  annealing 
work  demands  a  complete  change  of  structure — which  is  obtained 
only  above  the  upper  critical  range  (Ac3). 

Temperature  Difference  Instruments. — American-made  instru- 
ments for  determining  the  critical  ranges  of  steel  are  based  either 


418 


STEEL  AND   ITS  HEAT  TREATMENT 


upon  a  temperature  difference  basis,  or  upon  a  direct  record  of  a 
single  instrument.  The  method  used  by  the  Leeds  &  Northrup 
apparatus,  typical  of  the  first  class,  involves  the  following  points: 

Two  bodies  are  heated  together  in  the  same  furnace,  the  one 
being  the  steel  under  test  and  the  other  being  a  body  which  will 


Q&f.   —  .3 
Phos,=  .033 
Mng.  —  .700 
Sin.    =  .253 
Sir.     =  .029 


Heating.Curve  Cooling  Curve 

Abscissae — Temperature  Differences  between  Sample  and  Non-recalescing  Body. 
FIG.  222. — Transformation  Curves.     (Leeds  &  Northrup  Co.) 

heat  uniformly  without  undergoing  any  changes.  If  the  bodies 
are  in  sufficiently  close  contact  they  will  heat  at  the  same  rate  and, 
barring  changes  in  one  which  do  not  occur  in  the  other,  will  remain 
equal  in  temperature.  When,  however,  the  steel  undergoes  an  inter- 
nal change  involving  absorption  or  liberation  of  heat,  its  temperature 
changes  relatively  to  the  other  body  and  a  temperature  difference  is 
set  up  between  the  two.  Hence  the  apparatus  for  the  location  of 


PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS    419 

critical  points  by  this  method  is  designed  to  do  two  things:  first, 
to  measure  the  temperature  of  the  sample;  second,  to  indicate  the 
temperature  relationship  between  the  sample  and  the  unchanging 
body.  A  curve  using  temperatures  as  ordinates  and  temperature 
differences  as  abscissae  is  the  best  way  of  making  use  of  the  results. 

Temperature  Difference  Records. — Fig.  222  is  a  reproduction  of 
such  a  plot.  From  the  start  of  the  test  until  1205°  the  temperature 
difference  is  small  and  constant.  When  the  temperature  of  1369° 
is  reached  a  sudden  increase  in  the  temperature  difference  takes 
place,  the  Acl  range.  As  soon  as  this  sudden  change  ceases  (i.e., 
transformation  is  completed),  the  sample  and  the  unknown  begin 


FIG.  223. — Critical  Range  Curve  on  a  Direct-reading  Apparatus.  Carbon, 
0.44  per  cent.;  Manganese,  0.53  per  cent.;  Phosphorus,  0.035  per 
cent.;  Sulphur,  0.025  per  cent.;  Silicon,  0.028  per  cent. 

to  equalize  in  temperature  and  the  record  of  their  decreasing  differ- 
ence follows  a  typical  cooling  curve  between  1380°  and  1455°,  except 
at  about  1407°,  where  the  Ac2  transformation  begins  to  affect  the 
record.  At  1407-1410°  this  Ac2  change  is  completed.  Again  at 
1450°  there  is  a  departure  from  a  smooth  curve;  this  is  the  beginning 
of  the  third  transformation,  which  transformation  is  not  completed 
until  about  1495°.  This  is  the  Ac3  transformation.  On  cooling, 
the  reverse  takes  place,  except  that  the  two  upper  points  occur 
closer  together  and  appear  as  one.  The  lowest  range  is  clear  cut. 

Direct-reading  Instruments. — Fig.  223  shows  a  record  obtained 
from   a   Bristol   instrument.     Leaving   aside    a   discussion   of   the 


420  STEEL  AND  ITS  HEAT  TREATMENT 

scientific  pros  and  cons,  the  three  main  objections  to  this  class  of 
instrument  are:  (1)  the  small  area  covered  by  the  record,  involving 
less  accuracy;  (2)  lack  of  that  degree  of  sensitiveness  which  is 
necessary  to  bring  out  the  upper  critical  ranges;  and  (3)  a  curve 
showing  direct  temperatures  instead  of  temperature  difference. 

Practical  Method  for  Determining  Critical  Ranges. — For  plants 
which  have  to  determine  the  critical  ranges  but  infrequently  less 
costly  apparatus  may  be  used.  The  outfit  should  consist  of  a  thermo- 
couple made  of  small  wires  so  as  to  respond  quickly  to  any  slight 
variation  in  temperature;  the  necessary  leads;  and  a  sensitive 
milli voltmeter  or  pyrometer  with  a  finely  divided  scale.  This 
instrument  may  also  be  used  as  a  standard,  or  checking  instrument, 
for  calibration  work.  The  specimens  to  be  tested  should  be  small 
so  as  to  heat  uniformly  and  quickly.  These  may  be  either  a  small 
cylinder,  say  f  in.  diameter  by  If  in.  long,  or  duplicate  pieces  each 
1J  in.  long  by  f  in.  wide  by  J  in.  thick.  In  the  former  case  the  end 
of  the  couple  is  inserted  in  a  small  hole  drilled  through  the  axis  of 
the  cylinder  to  a  depth  of  about  \  in.;  in  the  latter  case  the  pieces 
are  clamped  together,  one  on  either  side  of  the  end  of  the  thermo- 
couple so  as  to  form  a  tight  contact.  The  specimen  is  then  heated 
in  any  convenient  manner,  readings  being  taken  every  few  seconds 
as  the  critical  ranges  are  reached.  When  the  indicated  temperature 
is  well  above  the  upper  critical  range,  the  specimen  is  removed  from 
the  heat,  allowed  to  cool  not  too  rapidly,  and  readings  taken  to 
obtain  the  Ar  ranges.  The  temperature  readings,  or  difference  in 
readings,  should  then  be  plotted  against  the  time  to  obtain  the 
necessary  curves. 


PYROMETERS  AND  CRITICAL  RANGE  DETERMINATIONS  421 


TEMPERATURE  CONVERSION  TABLE 

BY  DR.  LEONARD  WALDO 
Reprint  from  Metallurgical  and  Chemical  Engineering. 


c.  ° 

0 

10 

20 

30 

40    50 

60 

70 

80 

90 

-200 
-100 
-   0 

F.  ° 
-328 
-148 
(-  32 

-346 
-166 
+  14 

F.  ° 
-364 
-184 
-   4 

F.  ° 
-382 
-202 
-  22 

F.  ° 
-400 
-220 
-  40 

F.  ° 
-418 
-238 
-  58 

F.  ° 
-436 
-256 
-  76 

F.  ° 
-454 
-274 
-  94 

F.  ° 

-292 
-112 

F.  ° 

-310 
-130 

0 

32 

50 

68 

86 

104 

122 

140 

158 

176 

194 

C.  ° 

F.  ° 

100 
200 
300 

400 
500 
600 

700 
800 
900 

212 
392 
572 

752 

932 
1112 

1292 
1472 
1652 

230 
410 
590 

770 
950 
1130 

1310 
1490 
1670 

248 
428 
608 

788 
968 
1148 

1328 
1508 
1688 

266 
446 
626 

806 
986 
1166 

1346 
1526 
1706 

284 
464 
644 

824 
1004 
1184 

1364 
1544 
1724 

302 

482 
662 

842 
1022 
1202 

1382 
1562 
1742 

320 
500 
680 

860 
1040 
1220 

1400 
1580 
1760 

338 
518 
698 

878 
1058 
1238 

1418 
1598 
1778 

356 
536 
716 

892 
1076 
1256 

1436 
1616 
1796 

374 
554 
734 

914 
1094 
1274 

1454 
1634 
1814 

1 
2 
3 

4 
5 
6 

7 
8 
9 

10 

1.8 
3.6 
5.4 

7.2 
9.0 
10.8 

12.6 
14.4 
16.2 

18.0 

1000 

1832 

1850 

1868 

1886 

1904 

1922 

1940 

1958 

1976 

1994 

1100 
1200 
1300 

1400 
1500 
1600 

1700 
1800 
1900 

2012 
2192 
2372 

2552 
2732 
2912 

3092 
3272 
3452 

2030 
2210 
2390 

2570 
2750 
2930 

3110 
3290 
3470 

2048 
2228 
2408 

2588 
2768 
2948 

3128 
3308 
3488 

2066 
2246 
2426 

2606 
2786 
2966 

3146 
3326 
3506 

2084 
2264 
2444 

2624 
2804 
2984 

3164 
3344 
3524 

2102 

2282 
2462 

2642 
2822 
3002 

3182 
3362 
3542 

2120 
2300 
2480 

2660 
2840 
3020 

3200 
3380 
3560 

2138 
2318 
2498 

2678 
2858 
3038 

3218 
3398 
3578 

2156 
2336 
2516 

2696 
2876 
3056 

3236 
3416 
3596 

2174 
2354 
2534 

2714 
2894 
3074 

3254 
3234 
3614 

F.  ° 

C.  ° 

1 
2 
3 

4 
5 
6 

7 
8 
9 

10 
11 
12 

13 
14 
15 

16 
17 

18 

.56 
1.11 
1.67 

2.22 
2.78 
3.33 

3.89 
4.44 
5.00 

5.56 
6.11 
6.67 

7.22 
7.78 
8.33 

8.89 
9.44 

10.00 

2000 

3632 

3650 

3668 

3686 

3704 

3722 

3740 

3758 

3776 

3794 

2100 
2200 
2300 

2400 
2500 
2600 

2700 
2800 
2900 

3812 
3992 
4172 

4352 
4532 
4712 

4892 
5072 
5252 

3830 
4010 
4190 

4370 
4550 
4730 

4910 
5090 
5270 

3848 
4028 
4208 

4388 
4568 

4748 

4928 
5108 
5288 

3866 
4046 
4226 

4406 
4586 
4766 

4946 
5126 
5306 

3884 
4064 
4244 

4424 
4604 

4784 

4964 
5144 
5324 

3902 
4082 
4262 

4442 
4622 
4802 

4982 
5162 
5342 

3920 
4100 
4280 

4460 
4640 
4820 

5000 
5180 
5360 

3938 
4118 
4298 

4478 
4658 
4838 

5018 
5198 
5378 

3956 
4136 
4316 

4496 
4676 
4856 

5036 
5216 
5396 

3974 
4154 
4334 

4514 
4694 
4874 

5054 
5234 
5414 

3000 

5432 

5450 

5468 

5486 

5504 

5522 

5540 

5558 

5576 

5594 

3100 
3200 
3300 

3400 
3500 
36CO 

3700 
3800 
39CO 

5612 
5792 
5972 

6152 
6332 
6512 

6692 
6872 
7052 

5630 
5810 
5990 

6170 
6350 
6530 

6710 
6890 
7070 

5648 
5828 
6008 

6188 
6368 
6548 

6728 
6908 
7088 

5666 
5846 
6026 

6206 
6386 
6566 

6746 
6926 
7106 

5684 
5864 
6044 

6224 
6404 
6584 

6764 
6944 
7124 

5702 
5882 
6062 

6242 
6422 
6602 

6782 
6962 
7142 

5720 
5900 
6080 

6260 
6440 
6620 

6800 
6980 
7160 

5738 
5918 
6098 

6278 
6458 
6638 

6818 
6998 

7178 

5756 
5936 
6116 

6296 
6476 
6656 

6836 
7016 
7196 

5774 
5954 
6134 

6314 
6494 
6674 

6854 
7034 
7214 

C.  ° 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

EXAMPLES:    1347°  C.     2444°  F. +12°.6  F.  =  2456°.6  F.:  3367°  F.  =  1850°  C. +2°.78  C.: 
1852°.78  C. 


INDEX 


Abrasion,  resistance  to,  14 

Abrasive  wear  in  Mn  steels,  346 

Air  control,  182 

Air  cooling,  42,  58,  249,  276 

Air  hardening,  246,  249 

Allotropic  ferrite,  29 

Alloy  steel: 

hard  spots  in,  51 

necessity  for  heat  treatment,  1,  258 
Alpha  ferrite,  29 
Alternating  impact  tests,  10 
American  gas  furnace  process,  147 
Animal  charcoal.    See  Charcoal. 
Annealing.     See  Ch.  III. 

air  cooling  after,  58 

commercial,  62,  248 

definition  of,  39 

effect  of,  44 

elemental  considerations  in,  39 

furnace  cooling  after,  57 

hyper-eutectoid  steels,  254 

length  of,  48 

pit,  58 

rate  of  cooling  after,  53,  57 

rate  of  heating  in,  46 

size  of  object,  58 

slow  cooling  after,  58 

special  methods,  58,  59 

temperature  for,  47,  231,  234,  238, 
247 

time  experiments,  51 

vs.  toughening,  110 

wire,  398 
Armor  plate,  319 
Arrangement  of  charge,  214 
Ar  ranges,  32 

Atmosphere  in  furnace,  189 
Austenite,  27,  67,  231,  348 


Automatic  furnaces: 

for  die  blocks,  373 

for  shrapnel,  249 
Automobile  steel,  1,  234,  329 
Axles,  1,  8,  9,  90,  236,  241,  243,  245, 
247,  315,  329,  340 

B 

Ball-bearings,  300 

Ballistic  tests,  14 

Barium  carbonate,  127,  133,  137 

Baths: 

heating,  75 
.  salt,  76 

tempering,  99 
Best  case,  162,  165 
Beta  ferrite,  31 
Bit  steel,  394 
Bolts,  carburizing  of,  141 
Boring,  hollow,  88 
Box  annealing,  61 
Boxes  for  carburizing,  140 
Brains,  purchasing  of,  184 
Brine,  80 
Brinell  hardness.     See  Hardness. 

of  carbon  steels,  229 

of  chrome-nickel  steels,  318,  320 

of  chrome-vanadium  steels,  337 

of  nickel  steels,  290 
Brittleness,  3,  7,  8,  9,  110,  233,  240, 
246,  268,  270,  271,  275,  295,  306, 
344,  353 

Stead's,  61 
B.T.U.  values,  175 
Burners,  186 

C 

Calcium  chloride  for  quenching,  81 
Calibration  of  pyrometers,  415 
Capacity  of  the  steel,  106 


423 


424 


INDEX 


Carbides,  296,  304,  335,  348 
Carbon : 

concentration  of,  122,  271 

direct  action  in  carburization,  114 

maximum  in  case,  165,  276 

plus  carbon  monoxide,  124 

solution  of  in  carburization,  125 
Carbon  content: 

for  tools,  358 

influence  of,  3,  110 

in  manganese  steels,  345 

in  nickel  steels,  264 
Carbon  monoxide,  115,  117,  122 
Carbon  steel: 

under  0.15  per  cent.,  230 

0.15-0.25  per  cent.,  232 

0.25-0.35  per  cent.,  236 

0.35-0.40  per  cent.,  240 

0.45-0.60  per  cent.,  246 

over  0.60  per  cent.,  253 
Carbonates,  116 
Car  bottoms,  219 
Carburization : 

boxes,  140 

carbon    monoxide    plus    hydrocar- 
bons, 122 

carbon  plus  carbon  monoxide,  124 

depth  of  penetration,  126 

effect  of  chrome,  297 

gas  process,  147 

heat  treatment  requirements,  154 

object  of,  112 

of  chrome-nickel  steels,  308 

of  nickel  steels,  267,  270,  271,  273 

requirements  of,  112 

steel  for,  113,  232 

temperature  of,  125,  132,  133,  134, 
155 

with  carbon  monoxide,  115,  117 

with  simple  solid  cements,  135 

wood  charcoal,  115 
Case  carburizing.     See  Carburizing. 
Case  hardening: 

gears,  386-389 

maximum  efficiency  in,  165 

treatment  of  hyper-eutectoid  steels, 
156 

treatment  of  hypo-eutectoid  steels, 
155 


Case,  the  best,  162,  165 
Castings,  397 
Cellular  structure,  42,  53 
Cementite,  16,  24,  63,  114,  156,  163, 
165,  169,  253,  295,  301,  335,  391, 
402 

Centigrade  tables,  421 
Central  pyrometer  systems,  415 
Chamber,  height  of,  202 

twin-  furnaces,  226 
Changes: 

in  diameter,  366 

in  heating,  32 

in  length,  366 
Charcoal,  133,  135,  136 
Charge : 

height  of,  202 

influence  in  heating,  205 

influence  of  arrangement,  205 

placement  of,  75 
Charging,  39 

Chemical  composition,  effect  of,  1 
Chipping  chisels,  368 
Chisels,  298,  368 
Chrome: 

influence  in  carburization,  297,  308 

in  manganese  steels,  347 

vs.  silico-manganese,  351 
Chrome  steels: 

general  characteristics,  295 

0.5  chrome,  low  carbon,  296 

0.5  chrome,  0.35-0.50  carbon,  297 

0.5  chrome,  over  0.50  carbon,  297 

1.0  chrome,  300 

2.0  chrome,  302 

high  chiome,  302 
Chrome-nickel  steels: 

carburization,  309 

gears,  390 

heat  treatment,  309 

low  Cr,  low  Ni,  310 

0.5  Cr,  2.5  Ni,  322 

0.6  Cr,  3.5  Ni,  321 

0.75  Cr,  3.0  Ni,  322J 

1.0  Cr,  1.75  Ni,  322 

l.SCr,  3.5  Ni,  319 

Mayari,  329 

special  analyses,  327 

vs.  chrome-vanadium,  306,  335 


INDEX 


425 


Chrome-vanadium  steels.    See  Vana- 
dium. 

Circulation  for  cooling  oil,  84 
Classification  of: 

gear  steel,  386 

heat  treatment  after  carburization, 
155 

nickel  steel-,  258 
Coal  furnaces,  218 
Coffin  process,  244 
Cold  crystallization,  231 
Cold-end  temperatures,  410,  412 
Cold  rolls,  302 
Cold  rolling,  38 

vs.  strength,  6 

Cold  work,  effect  on  structure,  38 
Color  chart,  369 
Colors  in  tempering,  97 
Combustible  mixture,  177 
Combustion,      furnace      atmospheres 

from,  189 

Commercial  annealing,  39,  62,  248 
Commercial  data  in  carburization,  135 
Commercial  ratio  of  chrome  and  nickel , 

307 

Compensation  of  pyrometers,  412 
Compressed  air  in  quenching,  85 
Compressive  strength,  5 
Conservation  of  heat,  222 
Contact  couples  in  heating,  53 
Continuous  furnaces,  249,  252,  373 
Contraction  in  hardening,  88 
Contraction  of  area,  5 
Conversion,  temperature,  421 
Cooling,  after  annealing,  53,  231,  346 
Cooling  the  oil  bath,  82 
Cooling  the  water  bath,  82 
Corrosion,  295 
Couples,  409 
Cracking,  87,  246. 
Crank  shafts,  1,  329 
Critical  ranges,  25,  417 

changes  at,  41 

effect  of  manganese,  345^ 

effect  of  nickel,  258,  265~ 

heating  over  the,  42 

merging  of,  31 

of  chrome  steel,  295,  299 

of  chrome-nickel  steel,  309,  327 


I  Critical  ranges  of  high-carbon  steel,  253 
of  hyper-eutectoid  steel,  62 
of  manganese  steel,  350 
of  tool  steel,  363 

Cutters,  378 

Cyanide  hardening,  149 

Cyanides  in  carburization,  117,  139 


Dead  soft  steel,  230 
Decalescence,  417 
Deck  plate,  309,  319 
Depth  of  penetration,  272 
Design  of  furnace,  186 
Determination  of  critical  ranges,  417 
Diameter,  effect  on  tests,  229 

changes  in,  366 
Die  Blocks,  298,  369 
Dies,  247,  382. 
Differential  hardening,  82 
Diffusion,  44,  50,  232,  267 
Distortion,  364 
Distribution  of  carbon,  126 
Door  heights,  200 
Double  annealing,  59 
Double  carbide  steel,  304 
Double  quenching,  94,  163,  237,  254, 

276 

Double  regenerative  quenching,  166 
Drawing  dies,  302 
Drawing  of  wire,  398 
Drilling,  hollow,  88 
Drills,  298,  376 
Drop  tests,  8 

Ductility,  5,  7,  106,  257,  306 
Duplex  process,  329 
Duplication  of  results,  2,  107 
Dynamo  sheet  iron,  352 
Dynamic  strength,  2,   110,  236,  306, 

307,  319,  337 

E 

Effect  of: 

chrome,  295,  306 
manganese,  344 
mass,  286,  322,  330,  363 
nickel,  257,  306 
silicon,  350 
vanadium,  335 


426 


INDEX 


Elastic  limit,  3,  5,  41 
Electricity: 

atmospheres  with,  190 

for  heating,  191 
Electromagnets,  352 
Elongation,  5,  41 
Endurance,  6,  10 
Enfoliation,  120,  271,  279 
Engine  forgings,  232,  234 
Engraved,  dies,  374 
Equalization,  44,  48,  267,  269 
Equalizing  action  of  carbon  monoxide, 

124 

Equipment,  pyrometer,  411 
Eutectoid  for  nickel  steel,  267 

steel,  17 
Expansion  in  hardening,  88 


Fahrenheit  tables,  421 

Failures  of  heat-treated  axles,  246 

Fatigue,  2,  6,  9,  236,  307 

Ferrite,  23,  29,  164,  257,  258,  296 

Ferro-cyanides,  127,  139 

Files,  298,  302,  379 

Fine-grain  annealing,  59 

Fire-ends,  409 

Five-ply  steel,  396 

Flanges,  232 

Flue  construction,  222 

Force,  2 

Forging,  406 

Forging   temperatures  for  tool   steel, 

363 

Fragility,  9 
Frames,  automobile,  1 
Fuel: 

cost  of  delivering,  180 

costs,  175 

efficiency,  177 

equipment,  181 

fluid,  the,  178 

oil,  182 

oil,  air  control  with,  182 

selection  of,  179 

supply,  181 

the  right,  177 

uniformity  of,  182 


Fuel: 

vs.  furnace  design,  209 

vs.  operations,  178 

vs.  product,  191 
Furnace: 

atmospheres,  189 

batteries,  226 

cooling  in  toughening,  108 

design,  186,  209,  215,  407 

equipment,  74,  185,  192 

guarantees,  196 

plans,  223 

temperature  of  in  heating,  46 

the  one,  195 
Furnaces : 

automatic,  250,  373 

car-bottom,  219 

carburizing,  142 

coal,  218 

continuous,  250,  373 

forge,  407 

general  considerations,  227 

muffle,  207 

overfired,  213 

perforated  arch,  212 

practical  notes,  on,  227 

semi-muffle,  209 

shrapnel,  250 

twin-chamber,  226 

underfired,  196,  220,  249 

unit  system,  225 


G 

Gamma  ferrite,  31 

Gases,  action  of,  in  carburization,  114 

regulating  the,  39 
Gears,    1,    141,   236,    247,    329,    351, 

386 

Grade,  in  tool  steel,  357 
Gradual  cements,  133,  139 
Grain  size,  268 

at  Ac3,  41 

beyond  Ac3,  33,  42 

by  different  rates  of  cooling,  42 

effect  of  work  on,  38 

in  carburization,  134 
Gun  barrels,  235 

forgings,  91,  241,354 


INDEX 


427 


Hardening: 
cyanide,  149 
definition,  of,  65 
differential,  82 
heating  for,  71 
nickel  steels,  267 
pack,  148,  152 
strains,  97 
superficial,  148 
temperature  for,  70,  232,  237,  241, 

253,  274,  286,  336,  370 
tool  steel,  362 
vs.  annealing,  67 
Hardness,  11,  230,  272,  290,  295,  318, 

320,  337,  360 

Brinell,  11.     Also  see  Hardness, 
cutting,  254 
due  to  chrome,  297 
scleroscope,     13,     170.      Also     see 

Hardness, 
wearing,  254 
Hard  spots,  51 
Heat,  quality  of,  188 
Heat  application,  40,  195,  249 
Heat  conservation,  222 
Heat  reservoir,  201 
Heat  treatment: 
definition  of,  25 
growth  of,  1 
necessity  for,  1 
Heating: 

changes  on,  32,  40,  65 

costs,  173 

distinctive  conditions  in,  173 

factors,  174 

for  carburization,  143 

for  forging,  406 

for  hardening,  71 

for  tools,  367 

in  lead,  379 

in  salt,  76,  168 

influence  of  chrome  in,  297 

large  sections,  46 

length  of,  268,  370 

prolonged,  306 

rate  of,  46,  51,  52 

uniform,  196 

unit,  standard,  174 


Heating: 

with  electricity,  191 
Height  of  chamber,  202 
Height  of  charge,  202 
High-carbon  case,  treatment  of,  160 
High-speed  steel,  353 
High  temperature  carburization,  134 
High  temperatures,  effect  on  springs, 

391 

Hollow  boring,  88 
Hollow  tools,  383 
Hot-ends,  410 
Hot  work,  effect  of,  38 
Human  element,  39,  74,  183,  237,  245, 

406,  408 

Hydro-carbons,  118,  122 
Hyper-eutectoid  steel,  17 

annealing  of,  62 

zones  in  carburization,  120 
Hypo-eutectoid  steel,  17 

annealing  of,  40 


Impact  strength,  2,  8,  9,  106,  134,  170 

Impurities  in  carburization,  114 

Influence.     See  Effect. 

Intensifies,  337 

Intermediary  types  of  carburized  zones, 

122 
Interrupted    regenerative    quenching, 

168 


Jar  steel,  298 


Knives,  298 


K 


Laminations,  257 

Lead  baths,  75,  102,  379 

Ledges,  197 

Length,  change  in,  366 

of  heating,  39,  370 
Levers,  232 
Liquation,  129,  272 
Locomotive  axles,  243 
Low-temperature  carburization,  133 


428 


INDEX 


M 

Machine  parts,  240 
Machinery  steel,  232 
Machining: 

effect  on  structure,  38 

quality,  257 
Magnet  steel,  353 
Magnet,  use  in  hardening,  72 
Manganese: 

in  carburization,  113 

on  hardening,  95 

on  machining,  231 

steels,  344 
M'artensite,  68 
Martensitic  steels,  258,  302 
Mass,  influence  of,  202,  229,  286,  287, 

322,  341,  363 
Mayari  steel,  329 
Mechanical  mixture,  16 
Mechanical  work,  effect  in  annealing, 

48,  59 

Microscope,  use  of,  44 
Microstructure: 

of  high-carbon  steels,  255 

of  nickel  steels,  259 
Milky-ways,  50 
Milling  cutters,  378 
Millivoltmeters,  410 
Mineral  hardness,  306 
Mixed  cements,  272 
Molybdenum  steels,  353 
Motion  in  hardening,  74 
Muffle  furnaces,  207 

N 

Natural  alloy,  329 

Natural,  steel  in  the,  1,  3 

Navy  specifications  for  tool  steel,  360 

Network,  34,  42,  53,  63 

Nickel: 

effect  on  physical  properties,  264 

influence  of,  267 

influence  on  critical  ranges,  265 
Nickel  steels: 

2  per  cent.,  264,  274 

3.5  per  cent.,  274,  276,  310,  315 

5  per  cent.,  264,  267,  268,  276,  280, 
291 


Nickel  Steels: 

10  per  cent.,  264 

25-35  per  cent.,  264,  291 

carburization  of,  270 

for  gears,  360 
Nickel-chrome     steel.       See   Chrome 

nickel. 

Nickel-vanadium  steel,  342 
Nitrogen  in  carburization,  116 
Normalizing,  63 
Nuts,  232 


Obstructing  agents,  57,  258 

Oil.     Also  see  Fuel  Oil. 
quenching  speed  of,  78 
vs.  water  for  hardening,  243 

Oil  baths,  82,  101 

Oil  burners.     See  Burners. 

Oil  tempering,  80 

Oil-tempered  gears,  386,  389 

Oil-well  bits,  394 

Operators,  value  of,  184 

Oscillating  temperatures,  132 

Osmondite,  67 

Overfired  furnaces,  231 

Overheating,  72,  402 

Oxidation,  protection  from,  371 

Oxygen,  action  in  carburization,  115 


P 

Pack  hardening,  148,  152 
Packing  for  carburization,  141 
Patenting,  402 

Pearlite,  16,  23,  27,  56,  110,  258,  265 
Penetration,  depth  of,  126,  272 
velocity  of  with  chrome,  297 
Perforated  arch  furnaces,  212 
Phosphorus,  3 

Physical  properties  at  Ac3,  41 
Pins,  232 
Pit  annealing,  58 
Polyhedral  steels,  259 
Potentiometers,  410 
Preheating,  40,  370 
Process  annealing,  399 
Producer  gas,  181 


INDEX 


429 


Prolonged  heating  of  nickel  steels,  271 
Propeller  shafts,  329,  354 
Protection  of  steel,  60,  371 
Protective  deck  plate,  309,  319,  324 
Punches,  382 

Punching,  effect  on  structure,  38 
Purchasing  brains,  184 
Pyrometers,  408 

standardization,  413 

use  of  contact  couples,  53 

Q 

Quality  of  heat,  188 
Quality  of  product  vs.  first  cost,  173 
Quench-toughening,  111 
Quenching : 

after  tempering,  99 

baths,  77 

best  temperatures,  71 

double,  94 

manner  of,  90 

media,  77,  109 

special  methods,  80 

speed,  77 

tanks,  87 

water  for,  80 


Radiation  systems  for  cooling  oil,  84 

Eate  of  cooling,  39,  248 

Rate  of  heating,  39 

Razors,  298 

Reamers,  382 

Recalescence,  417 

Reduction  of  area,  5,  41 

Refinement,  33,  39,  41,  65,  160,  231, 

232,  237,  238,  248,  401 
Regeneration,  159,  160,  180,  274,  280 
Relation  of  austenite  to  carbide,  347 
Relation  of  physical  tests,  10 
Requirements  of  gears,  386 
Resilience,  8 
Rifle  barrels,  354 
Rings,  383 
Rivet  sets,  384 
Roller  bearings,  300 
Rotary  bending,  6 
Rounds,  hardening  of,  91,  93 


S 

Safe  steel,  396 
Salt,  use  in  carburizing,  137 

standardization  of  pyrometers,  413 
Salt  baths,  76,  102,  168 
Sand  baths,  100 
Saws,  298,  385 

Scleroscope,  13,  130,  290,  318,  320,  337 
Screw  stock,  240 
Screws,  carburizing  of,  141 
Seams,  257 

Selection  of  pyrometer  equipment,  413 
Selection  of  tool  steel,  357 
Sensitiveness  of  manganese  steels,  344 
Sensitiveness       of       silico-manganese 

steels,  351 
Shafts,  90,  290 
Shock,  resistance  to,  275 
Shore-hardness.     See  Scleroscope. 
Shrapnel,  249 
Silicon  steels,  350,  352 
Silico-manganese  steels,  351,  390 
Size  of  section,  58,  229,  248 
Slow  cooling,  33,  42,  57,  67,  107,  157, 

167 

Soft  forging  steel,  236 
Solid  solutions,  27 

Solution  of  carbon  in  carburization,  125 
Sorbite,  56,  59,  69,  103,  110 
Specifications  for  tool  steel,  360 
Spheroidal  cementite,  63,  163,  402 
Spheroidal  ferrite,  164 
Spheroidalizing,  63,  163,  164,  254,  355 
Springs,  1,  391 
Static  strength,  1,  2,  24,  319 
Standard  heating  unit,  174 
Standardization  of  results,  12,  110 
Stead's  brittleness,  61 
Steam  hardening,  246 
Steel: 

burnt,  42 

castings,  397 

for  carburization,  113 

nature  of,  16 
Steering  parts,  1 

Stresses  and  strains,  2,  6,  39,  97,  107 
Structure,  definition  of,  25 
Structure  of  slowly  cooled  steel,  17 
Sudden  cements,  133,  139 


430 


INDEX 


Suddenly  applied  loads,  7 
Sulphur  diffusion,  143 
Summary  for  case  hardening,  169 
Superficial  hardening,  148 


Table  for  temperature  conversion,  421 
Tank,  size  of  quenching,  87 
Taps,  366 
Temper,  358 

colors,  97,  93 
Temperature : 

conversion  table,  421 

effect  on  grain  size,  33 

effect  on  network,  34 

relation  of  surface  and  interior,  53 
Temperature  of: 

annealing,  39,  47 

carburization,  128,  132,  155 

hardening,  70,  72,  274 

pack  hardening,  152 

quenching  bath,  77 

toughening,  104 
Tempered  axles,  244 
Tempering: 

color  for  tools,  361 

definition,  of,  96 

for  depth,  98 

gears,  389 

handling  material  in,  101 

methods,  100 

oil,  80 

plate,  100 

quenching  after,  99 

springs,  392 
Tensile  strength,  2,  41 

of  cementite,  24 

of  ferrite,  23 

of  pearlite,  23 
Testing: 

comparative  results,  229 

errors  in,  307 

purpose  of,  1 
Tests  from  center,  229 
Thermo-couples,  409 
Threading,  treatment  for,  231 
Tie  rods,  232 
Time  of  heating,  231,  409 


Tool  steel,  annealing  of,  59,  60 

proper  carbon  for,  358 

selection  of,  357 
Torsional  strength,  1,  5 
Tough-hardness,  295 
Toughening,  103 

high  vs.  low  temperature,  109 

range,  1,04 

temperature  vs.  mass,  330 

vs.  annealing,  110 

vs.  ductility,  106 

vs.  impact  strength,  106 
Toughness,  231,  360 
Transference  numbers,   12,  230,  290, 

318,  320,  337 

Transition  constituents,  56 
Troostite,  68,  96 
Tungsten  steel,  350,  353 
Twin  chamber  furnaces,  226 

U 

Underfired  furnaces,  220 
Underfiring,  196 
Uniform  heating,  196 
Unit  furnace  system,  225 
Uses  of  chrome  nickel  steel,  329 


Value  of  furnace  operator,  184 
Valve  stems,  232 
Vanadium,  effect  of,  335 
Vanadium  steels: 

gears,  390 

nickel,  342 

Type  A,  339 

Type  D,  340 

Type  G,  341 
Vault  steel,  396 
Velocity  of  penetration  with  chrome, 

297 

Vents,  198,  199 
Vibratory  stresses,  1 

W 

Warping,  89,  232 

Water  bath,  cooling  the,  82 

Water  quenching,  80,  246 


INDEX 


431 


Water  spray,  79 
Water  toughening,  349 
Water  vs.  oil  for  hardening,  243 
Wear,  1,  14,  240,  302,  346 
Welding  of  alloy  steel,  330 
Welding  properties  of  tool  steel,  363 
Well  bits,  298,  394 
Wire,  398 


Wood  charcoal,  115,  127 
Work,  effect  on  grain  size,  38 
Working  conditions,  influence  of,  179 
Working  strength,  4 
Works,  annealing,  399 


Yield  point,  4 


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